Please try the below steps in order to fix this issue.
Restart the system and boot through safe mode as administartor.
Once you reach the safe mode desktop, run a system restore.
the system restore should reverse any changes to the system that might have caused the issue to occour.
click start...in the search box type system restore
in the window that appears, restore computer to earlier date and time and click next
select a date may be a week or 2 back when system was running fine and click next
follow the onscreen steps
In case the system restore does not help, please boot through safe mode with networking and download and run the Microsoft Fix it Tool from the below link
Microsoft Fix it Tool
If any how you are unable to boot through safe mode itself, in that case you will be unable to perform the above 2 steps.
In that case please run a startup repair of windows 7 using the windows 7 disc.
Please click on the below link that has the steps listed for the repair process
Windows 7 Startup Repair
Please let me know in case issue persists or in case you face any difficulty