Well now, since the difficult job of removing the belt is out of the way, reach in and give that alternator pulley a good spin.
Does it spin freely or even at all? Any noise?
Now turn is slowly and pay close attention to what you feel.
Do you feel any thing?
If the bearing seized that could easily thrown the belt, and of course the inside V will be burnt somewhat. If the bearing is just bad but the pulley turns you should be able to feel some sort of grinding as you turn the pulley. The bearing could even be bad enough to hear the grind. If everything feels good and the is no noise at all perhaps the belt wasn't properly adjusted or the lock nut for the adjusting star came loose.
Keep us posted.
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72
'02 CLK320 Cabriolet - wifey's mid-life crisis
2012 VW Jetta Sportwagon TDI...at least its a diesel
Non illegitemae carborundum.