a) try completely restarting your computer, perhaps something failed to load properly one rare time, that may resume working properly next start.
b) recover from your backup?
(You do have a backup, right?)
In your backup you can find your bookmarks in a location similar to this:
C:\Documents and Settings\Scott\Application Data\Mozilla\Firefox\Profiles\npk7tr4k.default\bookmarkbackups
c) Don't abandon Firefox over this unfortunate episode in the adventure that is your life.
In future, routinely backup everything.
Some ways (there are other ways too):
1) Bookmarks / Organize Bookmarks / File / Export
2) Install the free
MozBackup tool and it''' make a compressed backup of your entire Firefox user profile, including all your bookmarks, other settings, etc.
Just to get you smiling again . . .
Edit: You might see if this is of any help if the bookmarks were, alas, truly deleted:
Free file recovery sw