Your files are probably safe on your Hard drive, regardless of whether your Mac will start normally or not.
Restart while holding down the Shift key. This does one cycle of Disk Utility ( Repair Disk ), then loads minimal extensions and comes up with a login screen (even if you normally auto-login).
Look around. See if anything seems different. Let us know if there is any change the next time you try to start normally.
It is possible that you have just missed this repair program: