2011 Air has no font and displays nothing but question marks.

[Guitar69Freak]

This happened the other day and I don’t know what I’m doing. Can’t read anything because there’s nothing to read. How can I reinstall the font on my Mac?

 

[weirdsmith]

It may be possible to troubleshoot this in place, but if that were my machine I would be wiping it and reinstalling from my most recent backup. That will probably be quicker and easier than trying to fix this directly. I would also be on the look out for hardware problems - your hard disk may be on the way out. Good luck.

 

[KeesMacPro]

I would make a bootable USB installer of Mojave, boot from it and erase the internal drive.
After a clean install you can migrate all data with Migration Assistant from a backup.