Hi! Check this out--> " Go to Start > Run and type in "MSCONFIG". Try using the diagnostic start up and reboot. If you still are running into the same problem, go back into safe mode, back into msconfig and selective start up. Try removing one check mark at a time and rebooting. Repeat as necessary. If you get it to boot, then the one that you unchecked has got the issue in it. You will then need to go into that tab and basically do the same thing."( courtesy of crushbone,TSG).
In safe mode, try to run a virus scan. An infection looks likely as your prob. started abruptly!
The last dreaded cause might be a damaged Hard Disk. Download Acronis drive Monitor:Acronis Drive Monitor Review - Acronis Drive Monitor Download - Softpedia