Hello @Madhusudhanan_G,
Check this out on how to get a new copy of the assignment on Cousera. (There is no way to just get back one cell)
The instructions will require you to rename your current notebook as the system cannot overwrite your existing notebook. After getting the new copy, you can then move your solution there, continue to work on the new notebook, and submit your work with the new notebook.
Cheers,
Raymond