2002 Saturn L200 fuel gage and immediate shut down -unless

Engine will start when car is cold but after warm up and turned off -, car starts but IMMEDIATELY shuts down and wont start again and gas gage reads empty on fuel -even when full - dome lite strobes(flickers) when key is turned on but car still won't start HELP < was given a test by mechanic -but they cannot help- cannot find problem they gave up please give us a start if u can give a place to begin to check and a electric diagram if possible thank YOU.
Lynn