lost ability to access modern screen, cortana and apps

I upgraded to windows 10 from windows 7. All was working fine. Now, I can't access the modern start screen, cortana or apps. The following things might have helped cause this: I changed my PCs name, I installed more ram, I created a local log in and there was an update just before this happened. I tried to use a restore point but the laptop said it failed. I have tried the powershell solution with no luck. I have tried creating a new user but it won't let me add a new user.

Can anybody help? thank you

There seem to be several causes for this, and unfortunately not all are easily solvable. Have you tried running "sfc /scannow" and "dism /online /cleanup-image /restorehealth" from an admin command prompt already? If you create a new user account and log in using that, do they work then? That's all worth a try, but the same happened to me and it turned out to be registry hive corruption (which occurred just from a non-destructive partition resize) and the only fix was a full reset (reinstall).

-edit- Sorry, just re-read your message and saw you couldn't create another user account. That sounds serious, unfortunately.

Thanks I'll try those options.