With the new information you gave you seem to have a hardware problem. What does the screen say once you start the computer?
Did you build the computer? If so, this could be a connection problem with the hard drive. Or your power supply is dying.
Do you smoke around the computer? The reason I ask is that your connections could be dirty.
How old is the computer?
On a side note, my dry cleaner's computer went down. Knowing that I worked avionics, he asked me what could be the problem? He thought it was the power supply going out and I agreed.
But I asked him when was the last time he cleaned his computer? He never has in the 5 years he owned it. I said before buying a new power supply, clean the computer using Isopropanol alcohol and compressed air. Re-seat all the components and see.
The next time I saw him, I asked what the problem was. He said I was right, it was a cleaning issue. Never got any free dry cleaning out of it though.
It could be that easy.
Let us know,
Your Hardware Guide.