Yes, please.
Relax, you are doing fine, learning the ropes. just post the output of efibootmgr, all of it.
BootCurrent: 0001
Timeout: 1 seconds
BootOrder: 0001,0000,000A,000C,000D,000B
Boot0000* Windows Boot Manager HD(2,GPT,b6580eef-999d-4ab2-8015-4ab1c6342124,0x109000,0x32000)/File(\EFI\MICROSOFT\BOOT\BOOTMGFW.EFI)57494e444f5753000100000088000000780000004200430044004f0042004a004500430054003d007b00390064006500610038003600320063002d0035006300640064002d0034006500370030002d0061006300630031002d006600330032006200330034003400640034003700390035007d00000000000100000010000000040000007fff0400
Boot0001* Manjaro HD(2,GPT,a266b8ac-5933-e94c-8189-5b8f4d205b0c,0x800,0x100000)/File(\EFI\MANJARO\GRUBX64.EFI)
Boot000A* UEFI OS HD(2,GPT,a266b8ac-5933-e94c-8189-5b8f4d205b0c,0x800,0x100000)/File(\EFI\BOOT\BOOTX64.EFI)0000424f
Boot000B* Hard Drive BBS(HD,,0x0)/VenHw(5ce8128b-2cec-40f0-8372-80640e3dc858,0200)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
Boot000C* CD/DVD Drive BBS(CDROM,,0x0)/VenHw(5ce8128b-2cec-40f0-8372-80640e3dc858,0300)0000474f00004e4fcb000000010000008b00410053005500530020002000200020004400520057002d0032003400420031005300540020002000200069000000050109000300000000010416008b12e85cec2cf040837280640e3dc85803007fff040002010c00d041030a0000000001010600030101010600010003120a000100ffff00007fff040001043e00ef47642dc93ba041ac194d51d01b4ce639004400300044004c0043003200300035003100390033002000200020002000200020002000200000007fff04000000424f
Boot000D* USB HDD BBS(HD,,0x0)/VenHw(5ce8128b-2cec-40f0-8372-80640e3dc858,0900)0000474f00004e4fdd000000010000009b005700440020003100300045004100560053002000450078007400650072006e0061006c00200031002e00370035000000050109000200000000010416008b12e85cec2cf040837280640e3dc85809007fff040002010c00d041030a000000000101060001080101060003000305060003007fff040001045200ef47642dc93ba041ac194d51d01b4ce635003700340034003200440035003700340033003400310035003500330034003300350033003700330034003300300033003600330033003300350000007fff04000000424f
I don’t see this matching with your UUID’s.
Another reason to restore the boot loader:
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=Manjaro --recheck
sudo update grub
and reboot.
I appreciate that, but not sure what to do about it. For context, this machine has been configured this way for four years (dual boot Win10/Manjaro) and has gone through numerous system updates and kernel updates with no issue. Whatever went wrong happened with the last update via pacman.
Ok, just saw your edit: will attempt to restore the boot loader.
You might be right, but it can be fixed.
That’s because that is the partuuid, and I forgot to ask for that. it’s probably fine though but we can check.
lsblk -o name,partuuid,uuid
But yeah, run the reinstall, it will never hurt.
Or even better, I think install-grub is in the stable repositories.
sudo pacman -Suu && sudo pacman -S install-grub
sudo install-grub
It does all that is done in that tutorial.
NAME PARTUUID UUID
sda
├─sda1 086593e0-c3b8-4693-9060-90168c8ffd59 572B674853889C17
└─sda2 1bbd8da0-5fb1-4347-9803-695b6726f8d0 1ae9a221-eae0-4e67-8492-409706963343
sdb
└─sdb1 6e90e1a7-01 48BBB7380819D800
sdc
└─sdc1 0cabd99a-01 3676FF0B76FECB1F
sdd
├─sdd1 5ed13eea-7c13-8d4b-be2f-26f6defb0d73 ac2409e9-b990-460c-9388-a744776a2fcc
└─sdd2 a266b8ac-5933-e94c-8189-5b8f4d205b0c C8EE-10AF
sde
└─sde1 e8900690-01 72004FFD004FC737
sr0
nvme0n1
├─nvme0n1p1 8164696e-fd19-4e98-8fdb-1819eba44d95 0EBACBFEBACBDFF7
├─nvme0n1p2 b6580eef-999d-4ab2-8015-4ab1c6342124 4ECC-3C42
├─nvme0n1p3 0a06886b-622a-4b6e-8aaa-768cc3bd581e
└─nvme0n1p4 5a5a89a6-a352-43bd-bf03-29e62c0b4c91 884CD28C4CD27502
nvme1n1
├─nvme1n1p1 cdcd3b86-18c9-4b0b-a59e-180e4242e313
└─nvme1n1p2 a4002bab-a425-4808-97b7-14c7e1838573 C6D47D10D47D0449
OK, here we go. Nevertheless, it’s worth trying to restore the boot loader.
Yepp, its fine.
Either do what I edited in into my previous post, or follow the link wollie posted and do that for efi.
When I tried that, the update part seemed to work normally, but sudo install-grub returned:
WARNING: EFI directory not found! Grub couldn’t be installed.
And…
sudo update-grub says “command not found”
… and entering just
sudo grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=Manjaro --recheck
provides what?
install-grub is a relatively fresh script, maybe not perfectly working in all cases, yet.
??!?
There is something VERY strange going on here then!!!
But disregarding THAT just our of curiosity, what do you have in your efi boot?
sudo tree /boot/efi
Happy new year btw, its 0.01 her now. 
Yeah, like philm says, sudo pacman -Syu && sudo pacman -S update-grub
Seems update-grub is not installed on the system.
This could mean you have maybe a problem due to not properly shutdown Windoze, previously and it had not released the ESP. Can you still boot into Windoze?
I don’t have the tree command either…the last update was…messy…I’m afraid a bunch of stuff got borked. The linux side is usable, but obviously something is wrong.
The grub-install that Wollie suggested just says:
Installing for x86_64-efi platform.
Installation finished. No error reported.
Yes, Win10 is behaving normally. I just choose that option from the GRUB menu and it starts as always.
Happy New Year to you both!
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sudo update-grub can be replaced by:
sudo grub-mkconfig -o /boot/grub/grub.cfg
Happy New Year! 
In Windoze Fast Startup is disabled?
because it probably did, but you also need to update your grub menu with update-grub
Or that. But…
Yeah, you should probably have opened with that…
So what DOES a sudo pacman -Syu return??
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Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-6.6-x86_64
Found initrd image: /boot/amd-ucode.img /boot/initramfs-6.6-x86_64.img
Found initrd fallback image: /boot/initramfs-6.6-x86_64-fallback.img
Found linux image: /boot/vmlinuz-6.1-x86_64
Found initrd image: /boot/amd-ucode.img /boot/initramfs-6.1-x86_64.img
Found initrd fallback image: /boot/initramfs-6.1-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
Found Windows Boot Manager on /dev/nvme0n1p2@/efi/Microsoft/Boot/bootmgfw.efi
Adding boot menu entry for UEFI Firmware Settings ...
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
update-grub is still CNF;
Doing a normal update just says everything is up to date and there’s nothing to do.
Yepp, the missing update-grub is probably ONE problem. There is a hook each update (if needed) and if you did not replace that with grub-mkconfig -o /boot/grub/grub.cfg you will have to update your grub menu every time.
I can not help with that unless you just want to reinstall it with the commands I gave above.
But if you reboot now you should be booting into linux66