There is a utility called SuperDuper! (and Carbon Copy Cloner) that can make a disk image (.dmg) of a boot drive before doing any upgrades.
Some recommend this if problems surface after an update.
Backing up FMP databases is a critical step in avoiding disaster.
MLion upgrade should not have done anything to Fonts or your Font management (if you manage your fonts).
Do you know the fonts used in the interface? Are they listed in Filemaker? A reinstall of FMP may mean you will have to activate fonts in Filemaker that may not be automatically activated.
Layouts can become corrupted, objects can be corrupted, and databases can be corrupted.
If you have a backup, opening a copy of the backup may help diagnose the problem.