Sharepoint dlls overwritten in GAC mistakenly

I have overwritten the Microsoft.Sharepoint.dll and Microsoft.Sharepoint.Library.dll in Windows assembly cache mistakenly. Due to this My sharepoint site is not working. I am getting following error while accessing site:

Message: Exception occurred. (Exception from HRESULT: 0x80020009 (DISP_E_EXCEPTION)) - Exception: System.Runtime.InteropServices.COMException - StackTrace: at System.Runtime.InteropServices.ComTypes.IStream.Seek(Int64 dlibMove, Int32 dwOrigin, IntPtr plibNewPosition) at Microsoft.SharePoint.SPFileStream.get_Position() …. etc

Then I also tried to replace dll from Common files>Microsoft Shared but it could not work also.

Can anyone help me to get rid from this issue?