Yeah, follow the advice you have been given here. Whacking the top of the case is not likely to hurt it, beyond what is already damaged, but it is a common remedy that might temporarily resolve this particular problem. What whacking it does is temporarily make contact with a weak or cracked solder joint, until the high frequencies running across it vibrates it apart again or the Mac gets jostled. Most likely in your case whacking the Mac caused a permanent break in the solder, too far apart to to be mitigated by this rough approach. If you allow it to sit power up for several hours, the expansion from the heat might allow a good whack to make a connection. Other other hand, it could further weaken other joints. The only way to fix this is to go in and re-solder the suspect joints.
This more or less confirms what most of us have been thinking.