|The logo never comes up...|
Do you get the system page when you first boot up or just the blinking curser immediately?
If you do not get the system page or logo ...just the blinking curser immediately on start up then I think that your CMOS battery may be dead or the BIOS has corrupted.
If the system page comes up and then goes to the black screen then it is locked up on the harddrive or possibly a dislodged piece of hardware. More than likely the harddrive has started to fail you. This van be checked by entering the BIOS and enabling the S.M.A.R.T. drive program to see what it has to say about the harddrive
In reference to 11/05/2008
"So this is how liberty dies. With thunderous applause."
- George Lucas