There are a couple of different things that can cause this behavior. It could be random “bad weather in the cloud”, but if it is happening to you predictably running a given cell it is more likely that your problem is that the memory image of your notebook is getting too large. Are you sure you didn’t add any print statements in the inner loops that are causing the output to be more voluminous than necessary?
To recover, try doing “Kernel → Restart and Clear Output” and then “Save”. After that, you should be able to submit. The grader doesn’t need to see your outputs, only your code.