Not that anyone here would ever make this kind of error but just for arguments sake: If you were flying an aircraft that required switching between tanks (as opposed to a Cessna 172 that has a "both" option) and you ran a tank empty and the engine quit in flight what would you have to do to re-establish fuel flow to the engine. Assuming you had time/altitude, of course. Would it be as simple as switching the fuel tank and cranking the engine? Would you use the electric fuel pump if you had one? Prime the engine?