Login screen characters all question marks, login info isn't working
This looks like a font corruption issue. If you can boot into recovery mode and access Terminal you might be able to solve the problem by restoring the default system fonts.

First, to boot to recovery mode: Reboot your machine holding CMD + R until you see the Apple logo.

When you see the "Utilities" screen, choose "Utilities" from the menu, and then Terminal

Paste the following into Terminal:

fontrestore default

Press RETURN.

If this doesn’t fix the problem then paste the following after the prompt:

sudo atsutil databases -remove

Press RETURN.

Close terminal and restart normally.