I can't get any Illustrator files to open. I don't get an error message—just the spinning ball. I end up having to "force quit." Yesterday I uninstalled the program and reinstalled it and it worked perfectly, but this morning when I tired to open some files, same problems—won't open anything.
This sounds like a corrupt font every time you use it it corrupts the font cache. causing the launch problem.
I agree with Wade. The culprit is very likely an influence outside of Illustrator. In addition to fonts, I'd definitely also check on a generic level whether the files get damaged in any way. Indexing and backup e.g. with Time Machine may have damaged the files or restored an unusable file from a backup or worse even, may do something weird to the application executable itself.