i have a 1999 e 320 4matic and the belt fell off. what could have caused that and can i replace it myself or need a MB mechanic

Without knowing how adept you are at car repair it's hard to say if you can fix it or not.
I'd bet that the belt tightener failed and need to be replaced. It is a smooth pulley just about in the middle of everything belt-related, if you can spot that and see if it moves around easily or if you would see busted parts, that would be a start, or maybe post a pic of the front of the engine. On a scale of 1 to 10 in difficulty for an average mechanic, it would be around a 6 or 7.

__________________
Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

the V6 engine tensioner pulley is known to fail, I believe this site sells a whole kit with tensioner, idler and new serp belt.

it takes 30 minutes tops and a skill level of beginner - the only little aspect to take care of is the tightening of the mounting bolts, they take only 18 lb/ft torque at max. or more likely "lightly tight" for people who look like me or shrek.

__________________
2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)