It seems something is messed up with you kernel. It it recommended to always keep 2 kernels installed:
One to use and play with daily; and
one as backup in the case of the other one failing/giving problems. As is happening here.
My recommendation is to boot into the live ISO environment. Then reinstall the kernels, as you tried there, but a bit differently than you tried:
Once you have successfully booted into the live ISO environment, open a terminal and enter the chroot environment, as previously done using manjaro-chroot -a.
As you know, if success you are now root on your installation, and should be very careful. Now, I recommend installing a different kernel, 5.10 for example, as this is set to be the next LTS version:
mhwd -i linux510
If successful, reboot.
Once you have rebooted and everything’s working again, you can install another kernel as backup and/or troubleshoot how to reinstall 5.4.
That was my mistake, my apologies. You were right in adding -kernel to the command.
But let’s try updating the database first. From within the chroot environment, do:
pacman -Syyyu
Along with any software from the repositories, this should update the package database, which is what we’re after. Then try
sudo mhwd-kernel -li
again, to see what kernels are installed and also try:
sudo mhwd-kernel -l
to get a list of available kernels.
(You’ll probably have notice and wondered about the sudo prefix, seeing as we’re already root, but my opinion is “rather safe than sorry” and it hasn’t let me down yet.)
You can then try installing kernel 5.10 again, as previously advised:
I was unable to do anything with the issue so I ended up reinstalling. I really didn’t want to because I had some software that took a long time to setup, but luckily I had /home on a separate partition and was able to recover 95% of what I had.