Gas Line problem

I ran over a sewer cover and it ripped the whole muffler system off and I am getting that repaired now but before i put it in the shop i tried to put gas in it and it was acting like it was full already, I let it go because i was going to work but the next day my girlfriend was driving it and it stalled on her and cut off, it wouldnt start back up until later. My dad said it could be the due to the fact there is no muffler system, the AAA guy said it sounds like a fuel pump. I get the car back tomorrow so i will see if it starts up or not but any ideas off the top of your heads what it might be

some cars will not run if they detect fuel line problems

look under the car, see if the gas tank or lines are damaged