XPS 720 H2C beeps and won't boot up

Last Sunday my XPS 720 H2C was working in the morning. It sat powered up for several hours. When I went to use it later on, the monitor was blank and the fan was running very loud. I turned it off and left it until Monday. Now when I press the power button the fans power up , the power button stays green, the 1 flashes on and then off while the 3 and 4 stay on, then the 1 flashes on and off again. While this is going on a series of low beeps are heard. One beep, a pause, 3 beeps, a pause and then 2 beeps. The 3 and 4 stay on. And that's it, the fans stay running and nothing else. Naturally my warranty has just expired.