Fuel gauge?

Hi my car was in an accident someone ran into the back of me their car was a right off, mine was mended, then I had water leaking in the back, this was then repaired, but now the fuel guage showed 80 miles and the car stopped they have put pertrol in and it went ok, I don't fancy driving round and running out of petrol, no trouble before the crash, any idea why the fuel guage is playing up after their repair.

Thakn you.