Hi Rohit! Did you follow the steps here when you tried to refresh your workspace (scroll to the Refreshing your Workspace section) ?
That should get you a blank notebook. You can then copy your old solutions into it. Please make sure that all the cells run without error in this new notebook. Then click File -> Save and Checkpoint before trying to resubmit.
Hope this helps! If not, please send your latest notebook to me in a DM. Thanks!