Do you have windows 7 bootable disk or pendrive? If yes, boot from disk by entering BIOS and putting the drive or disk drive in first place in boot order. Now wait until the window appears where you can choose the language, time and currency... and click on next. In the next window click on the bottom left "repair your computer". Choose the installed operating system in the next screen and click on next.
In the next screen choose startup repair and wait until finished following the various windows and choose to repair and it will then restart the computer.
If this does not work you can follow the steps above and then try to choose a restore point or reloading an image if you have saved one.
If all else fails you will probably have to reinstall windows on the PC.