They didn't make sure the fuel level measuring thing was free when they dropped it back into the tank. So it's hung up at the spot where it would measure half full.
Who ever did the job, if it was a shop, needs to open it back up and set it straight. If you or a friend did it then you'll want to set aside the same amount of time it took you to do it the first time and do it again.
Should be fine to re-use the gasket if its not a crush one.