Hi Mark,
You can restart this by opening task manager or clicking the Ctrl+Shift+Esc at the same time when task manager popped up, find the windows explorer in the list click it and then restart. Then try clicking again the start button if it works.
If not, right click the start select terminal(admin) type this command Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"} enter and wait for it to completed then restart your device.