There are a myriad of problems that could have resulted in this.
A virus has shut down your system
Memory has gone bad
Last software you installed has corrupted the hard drive
When it is coming up – hit F8 (over and over again) till the menu comes up :
Windows Advanced Options Menu
Please select an option:
Safe Mode with Networking
Safe Mode with Command Prompt
Enable Boot Logging
Enable VGA mode
Last Known Good Configuration (your most recent settings that worked)
Directory Services Restore Mode (Windows domain controllers only)
Start Windows Normally
Return to OS Choices Menu
Use the up and down arrow keys to move the highlight to your choice.
See if you can get in safe mode.
IF NOT – repeat these steps, and instead of clicking on Safe Mode – Click on “Last Known Good Configuration”
IF THAT does not work:
Insert your recovery CD and see if you can boot up to that.
If THAT still does not work, you have a hardware problem, either memory chips (RAM) or your Hard Drive.