USB Ports Not Working

A few days ago I opened my computer to look at the hard drive. This is a Dell Dimension 5100C running Windows XP Professional. So I found the hard drive and did what I had to do, but when I turned my computer back on the mouse and keyboard did not work. I tried plugging them in to different ports, but there was no response. I tried restarting, but nothing happened, and since I can’t use the mouse there is really nothing else I can do. I know the mouse works because I have tried it in other computers. Both the mouse and keyboard are wireless. Please help me figure out what could have happened to my computer, since as far as I know I haven’t done anything to the inside.