I don't think this is the cause of your problem but it's no harm to check anyway. If Custom Function 4 is set to 1, you can no longer focus with the shutter button, unless you use one of the Auto (or Pic) modes, which over-ride custom function settings.
Do a system reset of the camera (not entirely sure which menu item that is, as I've never used it). If that doesn't work, just take out the batteries and leave them out for some time. This should also result in everything being reset.
Also, check that you have the latest firmware running and even if you do, it might be no harm to re-install it anyway.
- Thorsten.