What mistake did you make ?
If you are talking about you 'fixed' something using HJT that you shouldn't have then you can reverse the 'fix.' 1
Run HJT again.
2. on the right of the screen is a button, 'Config...' click that and click the Backups.
3. You will now see a list of items you 'fixed' with the tick boxes to the left, if you know what the problem was you can tick the item and click the Restore button on the right of the window.
But, we really need more information, so don't got jumping too soon.