Box with question mark shown for every character in OS X El Capitan

I would reinstall the OSX.

Ciao.

Given the desktop, I take it the OS is either El Capitan or Yosemite. First I'd try booting in Recovery Mode and see if that comes up ok. While in that mode, I'd run Disk Utility and do a Disk First Aid (Repair Disk on Yosemite) on the internal drive: the device and the volume. If on Yosemite, you can also Repair Disk Permissions. Quit and restart normally and see if things are fixed. If not, next to try would be starting up in Safe Mode. Wait till it finally finishes starting up and see if the problem is fixed, restart normally and see if the repair holds.

If not, then your system fonts are hosed and a reinstall is necessary. Restart in Recovery Mode and use the Install OS X option. Elect to install on the current system disk without erasing and your stuff should survive intact. If possible do make a backup beforehand.

Please 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.

Which of the two helped? The Disk Repair or the Safe Mode?