Hello @Xashax007,
For each exercise, there are two header lines like below:
which are likely also the lines missing in your submission, and you may either try to add them back, or, as you said, reset the whole notebook.
To reset, following these steps, and be reminded that in the process you will have to rename your current notebook before the system can get you a new copy, then you may copy any thing you want from the renamed one to the new one.
Cheers,
Raymond