This could be a serious problem. Turn it off, disconnect the hard disk and turn it on again. Normally the BIOS will tell you it can't find a hard disk. Then find your Windows 7 DVD, put it in, reboot again and see if it will boot from that.
Tell us what happens.
I have an Alienware x51 desktop with windows seven, recently my computer went into what I thought was sleep, my power button is lit and my front usb ports are putting out power because I use them to charge small items, other than that the computer is completely unresponsive. I have pressed my mouse buttons, pressed my space bar, unplugged the power supply and left it unplugged for several hours before plugging it back in, when I plug it back in I do not even need to press the power button and as soon as the cord is plugged in it goes straight to this sleep mode, the led lights on the sides are not lit and the fans are not running, I have held down the power button until it powers down and as soon as I press it back to this sleep mode. That's all I can get it to do. Someone please help.