In the last few weeks we have suddenly been unable to use our desktop computer at home. Every time the computer turns on a box appears on the monitor saying no VGA input and puts itself to sleep. What could the problem be and what could have caused it? Is it the monitor, cord or something else? We have tried everything to make it work. All of the cords and such are securely placed and there's no evident damage anywheres. The computer is 3 years old and an HP.