I can't log in to my MacBook Pro. The log-in screen now has question marks in place of letters and won't allow me to type in the password box.
The linked question covers the why of the fonts being messed up. The part of the system that renders the system fonts is damaged or intentionally changed (but perhaps without realizing the ramification of disabling the fonts until now.)
The fix is to boot to recovery HD and reinstall the core OS. That will replace the part of the system that has the needed fonts and not overwrite / erase your data like Applications and emails and photos and such.