What you do in System Restore is revert to a saved restore point. See How To Use System Restore in Windows for instructions on the process.
If no restore point exists on your computer, System Restore has nothing to revert to so the tool won't work for you.
If you're trying to recover from a major problem, you'll need to move on to another troubleshooting step.
The amount of space that restore points can take up is limited (see Restore Point Storage below) so old restore points are removed to make room for newer ones as this space is filled up. This allotted space can shrink even more as your overall free space shrinks, which is one of several reasons why I recommend keeping 20% of your hard drive space free at all times.
