Using an old archive that I have (with Maxwell's equations), the Greek letters display as squares. I tried to change something in the alphabet but even the fonts don't have any Greek letters (they appear like squares too), both Greek and iGreek letters packages.

Sounds like a package that's not installed, or corrupted. I still re-downloaded and reinstalled LibreOffice. I don't know what I have to install. The equations look fine in OpenOffice.org (I made the file with it). What should I do?