RE: After system restore: error message ‘A operating system wasn’t found’
If you have a set of recovery media (that you were prompted to make when you first turned on the system), you'll need that now. If you don't, I think you'll find only Windows 10 is an option now - Dell doesn't release recovery media for Windows 7 any longer.
You can alternatively purchase a recovery media set from Ebay, etc. for your system.