Please try one of those solutions :
1-First download windows 7.iso from this link : http://getintopc.com/softwares/operating-systems/windows-7-ultimate-free-download-iso-32-64-bit/
2-Burn ISO Image to a Blank DVD.
3-Put the disc in your optical drive and restart to boot from the DVD. You may have to change the boot settings in your BIOS if booting from a CD/DVD is not enabled.
4-On the Install Windows screen, make the appropriate selections for language, time, and keyboard, and then click Next. On the next screen, click “Repair Your Computer”. Do not click "Install now"
5-In “System Recovery Options”, select “Startup Repair”
6- check if your computer is working now.
Solution 2 :
Repeat the same steps, In “System Recovery Options”, select system restore
on the Restore system files and settings window. Choose the restore point that you want to use. You should use the restore point with a date when you still don't have the problem you are facing now.
Solution 3 :
Insert your Windows 7 Disc, After booting from the Windows Installation Disc, click “Repair Your Computer”.
Select Command prompt and type chkdsk drive: /f
then type : chkdsk drive: /c ( replace drive with your drive letter, example: D: , E: )
Let Windows finish checking and repairing your disk.
"repeat solution 3" but this time type : bootrec /rebuildbcd You will get a message saying ” The operation completed successfully” Restart your computer and check
solution 5 :
Format your computer ( All data on your hard drive will be lost)
Please follow the step by step instructions in this tutorial video : https://www.youtube.com/watch?v=mME9VLT7-xw
solution 6 :
If none of the solutions does not work, then it could be a hardware issue, please Replace your hard drive
Hope this help