Does RIGHT click at Start button still work ?
If yes, try this.................( or have you tried that already ? )
Right click at Start button > select Command Prompt (Admin) > type powershell > press Enter > wait a bit until you see....
PS C:\windows\system32>
At the flashing prompt, copy and paste this.........
Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}
> press Enter.
Ignore any error in red.
When finished, try Start Menu again.