After the most recent update on 8th March 2021, the Grub menu disappeared.
I managed to get the menu to reappear by running the following command
sudo nano /etc/default/grub
and then editing the grub file and changing the following
GRUB_TIMEOUT_STYLE="hidden"
to
GRUB_TIMEOUT_STYLE="menu"
sudo update-grub
sudo grub-mkconfig -o /boot/grub/grub.cfg
However both my Windows 10 (on the same SSD ) and my 2nd (backup) Manjaro installation on a second drive HHD do not appear in the Grub menu anymore. They are both on GPT partitioned drives with EFI boot.
After running os-prober I get the following
sudo os-prober
/dev/sda2@/EFI/Microsoft/Boot/bootmgfw.efi:Windows Boot Manager:Windows:efi
/dev/sdb5:Manjaro Linux (20.2.1):ManjaroLinux:linux
I can boot into both Windows 10 (SSD) and my 2nd Manjaro installation (HDD) if I choose the appropriate device from the BIOS, but they no longer appear in my Grub menu.
If anyone can help that would be greatly appreciated. This exact problem has also occurred on one of my separate machines which is much older, Windows 10 was lost from the Grub boot menu.