Instructions
Check the actual physical connection between the camera and at the computer. The USB cord can be especially sensitive where it comes out of the camera. Applying gentle pressure to or slightly bending the USB cord is sometimes enough to make a connection.
Replace the cord, if you can, and see if you can make a connection with the new cord. If you can, you have found your problem.
Check the battery. You need a little power to connect to the computer, and if the camera battery is totally drained, you will not be able to connect. A replacement battery is helpful here, but if you don't have one, just recharge the one you have and try again.
Restart your computer. Sometimes this will clear a software issue that is preventing a connection.
Try a different computer. If you can connect on the new computer, you know there is a problem with the prior computer. Try updating the drivers for the computer, using the link listed under Resources.