user account login pane fonts changed

Back up all data before proceeding.

Launch the Font Book application and validate all fonts. You must select the fonts in order to validate them. See the built-in help and this support article for instructions. If Font Book finds any issues, resolve them.

Start up in safe mode to rebuild the font caches. Restart as usual and test.

Note: If FileVault is enabled in OS X 10.9 or earlier, or if a firmware password is set, or if the startup volume is a software RAID, you can’t start in safe mode. In that case, ask for instructions.

If you still have problems, then from the Font Book menu bar, select

File ▹ Restore Standard Fonts...

You'll be prompted to confirm, and then to enter your administrator login password.

Also note that if you deactivate or remove any built-in fonts, for instance by using a third-party font manager, the system may become unstable.

If you don't already have a current backup, back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.10 ("Yosemite"), but the procedure is the same for OS X 10.7 ("Lion") and later.