There's no specific spot hitting the gas tank with a hammer will reliably restart a fuel pump. A mechanic resorting to that is extremely unprofessional and potentially dangerous. Hitting the fuel tank could damage it, causing leaks, or even creating a fire hazard. The temporary restart is likely due to a loose connection jarred back into place. This is not a solution, and the problem is much more serious than a simple whack.
The fuel pump itself is likely the issue, and it needs to be properly replaced, not brute-forced into working. The mechanic's actions could have further damaged the pump or other components in the fuel system. Do not attempt to hit the gas tank again.
To fix the problem, the fuel pump needs to be replaced by a qualified mechanic. They will access it (usually by dropping the fuel tank) and replace the faulty pump correctly.
