If there isn't anything important on it then you could do a Windows 8.1 Factory restore, that might give you back your system.
Use the following steps to reset Windows:
From the login screen, click the power icon in the lower right corner of the screen.
While holding Shift, click Restart.
Click Troubleshoot.
Click Reset Your PC.
Click Next.
The system will reboot and begin Preparing to Reset the PC.
At the Reset your PC screen, click Just remove my files.
Click Reset.