Did you change / install / upgrade anything on the system?
If you did none of this, it sounds as if you might have some kind of severe hardware problem. It could be that some device has gone bad, or is not getting started properly when the module that drives it is getting loaded, etc. etc.
Basically it can be just about anything. You have four options:
1. Use a rescue disk
If you have a rescue disk for your distro, start it up and see if you get the same behaviour if running in text mode
2. Use Windows
Counter-intuitive maybe, but I have often identified bad hardware this way. If the PC / laptop misbehaves in Linux, and you happen to have Windows installed, start Windows up and check if the same type of failure / problem occurs there. If it DOES, you know that you have a hardware problem.
3. Start in text mode
If your rescue disk works, edit /etc/inittab and set your starting runlevel to 3 (at least, this works on Fedora Core 6) so that your system starts in text mode and won't start Xwindows by itself. If it seems to be stable and working in text mode, try starting Xwindows by typing
in the text terminal. If your problems then start, you know at least that your trouble lies with the graphics sub-system or your x-windows installation, but the rest of your system should be good (since it works in text mode...)
4. Reformat and reinstall
This is the final, most extreme solution. If you have some kind of misconfiguration or failed auto-update causing the problem, this might solve it by reverting your system to known, stable, and working state.