If the worst happens, it is probably your motherboard. You then can take your old hard drive and stick it in a “conversion kit”. Your files (and programs!) can be accessed through another computer via a USB connection.
Do you see the BIOS on the screen when it comes on? Is the screen black? Can you put your hand behind the computer where the power supply fan is to see if it is coming on? Did you take the side off of the tower and see if the CPU fan is coming on when you push the start button? Just some things to check.