You may use the recover discs supplied to you by your vendor. In case they aren't then you have to reinstall Windows 7 removing the current system partition. Windows does not allow downgrading Windows 8 to Windows 7. A fresh install is advisable to avoid any conflicts and for smooth running.
What is the exact error that you are facing. If possible then take a snap of the monitor and post here so it will be better to see the error and suggest a solution accordingly.