Mac Pro Will Not Boot Up - No response Whatsoever!

Hi,

I purchased a Mac Pro sometime last year. Today out of the blue it's acting strangely. It will not boot up. I don't even get the Apple logo, or anything. No reaction or response from the monitor, keyboard, etc. Nothing. Just a black screen, as if the monitor is not even on.

I have tried switching monitors thinking that was the problem, but I've hooked up said monitors to two different computers, and they work fine, so I know that's not the problem.

When I press the on button on the tower, the LED light turns on, and the entire unit sounds like it's running, but nothing else happens.

I used the computer last night to watch some videos I downloaded into an external hard drive. I turned off the computer after I was finished. The external HD remained connected to the Mac Pro, though in standby mode.

Today the Mac Pro is not responding what so ever. I've disconnected the external hard drive and even unplugged the computer, but nothing happens.

I really appreciate any help I can get with this issue.