Here’s another recent thread about this error.
Did you add any print statements that make your output very large? That can cause the memory footprint of the notebook get too large. You can try getting a clean copy of the notebook by doing “Help → Lab Help → Get New Version”, if you can click that link. But that only replaces the notebook if it has been renamed, so it may not really help unless you can first get the notebook open and rename it.
Try the things suggested here and on the other thread. If none of that works, we may have to see if the course staff can reset your notebook for you.