My suggestions below are for windows 10 PC, not meant for WP.
Suggestions :
1. Right click at the Start button > click Powershell (Admin), or Command Prompt (Admin), whichever if that is on the list instead > copy and paste this command line ( it is best to do copy/paste to avoid typo)........
Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register"$($_.InstallLocation)\AppXManifest.xml"}
> Enter
See if that fixes the issue.
2. Run a system file check to look for corrupt files. It will try to do repair if any found.
At the same Powershell (Admin) window, copy/paste this.....
sfc /scannow
>Enter.
It will take quite awhile. Please be patient.
When completed, it will give you one of the following 3 reports: (paraphrasing)
a) No integrity violation.
b) Found corrupt files and repair successfully.
c) Found corrupt files but unable to repair some of them.
Please post back the result.
3. Create another admin user account > logon to it.
If Start Menu works fine in the new account, it indicates your old admin account is corrupted.
Use the new admin account for a few days to make sure.
If all goes well, logon to the old admin account > backup files you want to keep > delete the old admin account afterwards.