My computer suddenly stopped booting up. I just came back from a week long trip, and it was working fine before I left. When I power up the computer now, it seems to power up just fine (leds on, fans working), but then there's no output on the screen at all, and there's a noise that seems to be coming from the HD, like it is trying to read something. After the noise starts, it does 6 cycles and then stops. I'm not sure but I don't think it did this noise when the pc was still working. I tried using the onboard video card instead but it made no difference. What could be wrong with my computer?