what's the problem?

A serpentine belt has nothing to do with starting. Even when installed incorrectly or left off altogether the engine will start .

You either disconnected something unrelated to the serp belt ..or…..I would have to ask why you replaced the belt in the first place.

Ok so first question is the starter rotating the motor like it should and it just wont start or are you turning the key without getting any results. If its not turning over id first off hit the starter solenoid with a soft hammer a few good raps may dislodge a stuck solenoid. Now if the motor is spinning but not firing up my first place id look is the distributor wiring unless it runs coil packs then i would guess that you knocked the wires off the crank sensor or maybe even broke the crank sensor.

So, take the belt back off and see what is locked up? An alternator? An idler pulley? Water pump? Something has to be holding it up. I have seen alternators that have locked up and the belt snapped, someone put a new belt on, then it wouldn't even crank over. 

If it didn't start before replacing the belt, it won't start after replacing it, simply because the battery is still dead. A dead battery is a very common consequence of a broken serpentine belt as the alternator will stop charging the battery and will only start charging it after replacing the belt and starting the engine. 

Crank no start can be no fuel, no spark, or weak battery(crank will be slow and weak)

No Crank No Start with this you will just hear a click or rapid clicks which are the relays trying latching and unlatching

This can be bad starter motor or bad battery.