...do you have XP?....have you tried system restore? (most likely won't work but perhaps it's worth a try).
Also, as an experiment perhaps try reloading your version of mame (start with a fresh file ...no roms, art etc.). Then load up a few snaps. See if they show up OK....if they do, you've probably somehow changes a cfg. file somewhere.
I don't have a lot of computer experience but I've solved similar "oddities" like yours by simply reloading mame (and then moving all of my art, cab etc. files over to the new version).
Just drag all of the files over...it should only take about 10 min to be up and running again.
The MameMaster!