Re-seating the cards, literally means removing and refitting them. When you open your PC case, you will see them. PCI cards are removed by undoing one retaining screw, and if you have an AGP graphics card, this sometimes had a small clip at the back, which slides to release.
It's not really advisable to shake a PC, but it does indicate that a loose component or card may be your problem. Well, if it wasn't loose before, it probably is now!
While you're inside the machine, also check all the power connections to the drives, and make sure that the IDE cables are securely in place. These are the ribbon cables that run from your motherboard to the drives. I assume, when you said before that you have checked the connection, you meant the external ones.
Not knowing how much you know aboout tinkering inside a PC, I should also advise you to make sure the power is disconnected at the mains, and that to avoid static discharging from your body, and possibly damaging a component (although it's never happened to me), general advice is to earth yourself on a radiator, or something similar, before touching anything inside your PC. Don't go poking about too much with screwdrivers either. Just use them as necessary to undo retaining screws. Also advisable not work on a nylon carpet (static again).