If its a monitor you are referring to and are having problems with.
One bad or faulty lead connection can cause a computer to continue restarting on a cycle or to shutdown or fail to detect your hard drive. WARNING: Before you start troubleshooting remember that you are dealing with electricity that can KILL. http://www.kitchentablecomputers.com/static.php - rules
Test all power and data leads that attach to your hard drive IDE,SATA the leads from your (motherboard to your hard drive) make sure they have secure dust free connections and are not faulty if its a 40 pin flat ribbon type it will be the first to fail make sure all leads that are attached to your drives dvd\cd 3 1/2 inch floppy have secure connections and are not faulty even something as small as a faulty electrical fan and its lead can cause you problems computers need all power and data to continue through every working device and to have an end so any faulty leads will end up with a computer error
Make sure your graphics card is securely seated with no dust built up or in the socket if you remove your graphics check the socket to make sure its dust free restart your computer then reinstall it this should activate found new hardware wizard Hope this helps.