Try using a different browser.
You probably by any chance deleted anything which you shouldnt have, then click 
and reboot the lab environment, Update Lab. you should be able to get back the files if you have access to the course.
I joined it through company portal, I suppose I have access as I was able to enroll in new course today.
Tried that too, not able to submit. Is there any way to communicate with IT team or can I get new notebook someway?
ok you mentioned company portal, chances are that would be restricting your lab access.
to get a fresh copy, i already mentioned the step.
follow 1a. in the below link
I am pretty certain that’s it is not the case, because when I again tried with week2 assignment, it was saving my content:
But even after trying everything for week 3, it’s not working. Rebooted multiple times and done other stuff too. Seems like I am stuck.
Also, I noticed that if I am trying to download vanilla notebook without my answers then I am able to do so, but if I am doing my changes then I am not able to do so. Also autosave is failing in week 3. If possible, provide me email id where I can upload my answer file. Someone can run it against test cases so I can complete this course 
did you raise concern at the Coursera’s Learner help center?
If you are using a company portal, you may have to ask them to investigate this issue.
Speaking for myself, this is nothing I have ever heard or seen before.
Yes, they redirected me here too.
If you have a course access problem, the mentors cannot fix that.
You would need to raise the issue with the Coursera Help Center.
I was able to submit, I created new file of same name then copy pasted each python cell as upload file was not working(again I don’t know why). After that I was able to save and create checkpoints and submission was also working. Finally got the certificate 
Thanks @TMosh @Deepti_Prasad for taking that much pain, you guys were really helpful!
That’s very good news!