Please try what Kin has suggested and let us know if that changes the results.
If you are concerned that you may have damaged the notebook, there is a straightforward way to start over again with a clean copy, as described on this thread.
It would also help us to help you if you could give us more details about what you mean by not receiving feedback. Do you mean that the grader literally never returned any result at all? Or that it gave you 0 points for everything? If that latter, then it might shed some light if you could show us what it says when you click “Show grader output”.