Many of the most frequent issues surrounding problems associated with desktop computers are improper connections between the computer tower and its peripherals. When something is not working properly, such as the mouse or the keyboard, you should first verify that the item in question is properly connected.
There are two ends to every connection. The end of the cable connected to the computer tower, and the end of the cable connected to the keyboard, mouse, monitor or other device such as a printer.
Verify that both ends of the connections are secure and in the proper locations and that all devices have power to them.