New
#1
Two things to try.
1) Start> In search box type view installed fonts> Enter> Font settings on left> See if Hide fonts based on language settings is checked
2) Navigate to C:\Windows\System32\FNTCACHE.DAT and delete it. Reboot, and it will be created again.
A Guy
Try creating a new user account and see if it works there. The square is usually a "place holder" for when the correct fonts are not installed (I'm assuming you have Korean fonts as you said it worked until you rebooted), or corrupted (You might need to uninstall and reinstall). Let's see if a new user account works correctly.
User Account - Create
A Guy