PC not Booting. Only Says Compaq.

The Screen comes on and say’s compaq in big letters, and in the top left hand corner it does what it looks like a memory check and say’s 65550KB OK and then puts a cursor on the top right corner of the screen that sits there and blinks. I can’t get into safe mode or anything. I’ve tryed every cable, reconnected the cards and the processor and nothing works. I swaped hard drives and cables and still the same error. I tried swaping the memory and still the same thing just with a different memory number. Can anyone help?