grand prix wont start after serpetine belt broke

Chances are, the engine is out of time. You need to get the cam and crank back in sync. I don't know much about these newer cars, but the old school way was to get # 1 piston up and ensure that the distributor rotor is pointing at it. Remove #1 plug, insert soda straw, slowly and carefully rotate engine via small "bumps" of the starter until #1 is at the top of its stroke. I usually timed from there by ear, but you should probably use a timing light. As I said this is old school, and I think the idler pulley can be involved in the sync process on newer cars. If no one with more experience with newer cars does not answer maybe you can try my method. You do not want to run an engine out of sync as damage may occur. Good Luck.

If it is the external sepentine belt that is being referred to it does not drive the camshaft so having it fail cannot affect the timing. That belt only drives the accessories. If it is the internal timing belt we are talking about, then that is a different story entirely. It is critical that it be installed exactly to spec or the engine will indeed have problems. A wee bit more information would be helpful.

There were belt remnants behind the crankshaft pulley. I haven't removed pulley to check crankshaft sensor, but i,m hoping that is it.