windows 7 will not wake up

i am  running windows 7 on a dell desktop and suddenly the machine will not wake up... it does not respond to the mouse, keyboard or on/off button. i have removed the plug and let the power drain but when i turn it back on i hear the computer start up however the monitor says there is no signal from the computer and is still asleep. This machine has had no trouble with sleep for the past 2 years but i can't even check the settings because i can't seem to bring it out of the sleep state. How can i wake it up so i can check further?