Cell #9. Can't compile the student's code. Error: NameError("name 'mu' is not defined")

lab id:-

It appears you have a variable named “mu” in one of your functions that is not defined.

You may need to run all of the cells in the notebook as soon as you open it.

Can you say a bit more about when this issue occurs? You didn’t provide very many clues.

Today I opened my notebook, and all my code which I have written is no more.
I think my code has not been saved, because of which it might be given this error.

Now it is resolved.

Thank you.