Start Menu & Button not working in Windows 10

Before you proceed create a system restore point first so that you can revert back, if you find that the results are not what you expected.

1] Download and run the Windows 10 Start Menu Troubleshooter.

2] Run the following command prompt, as Administrator, to run the System File Checker.

sfc /scannow

Restart after the scan is over and see if it helped.

3] Repair Windows Image. Open a command prompt, as Administrator, and copy-paste the following and hit Enter

Dism /Online /Cleanup-Image /RestoreHealth

Restart after the scan is over and see if it helped.

At this point the start menu, for me, is usually working...

But if it is not, there are a couple more things you can try.

4] Make Start full screen and back. Enable Tablet Mode and Start Screen and then go back. See if this toggling has helped.

5] Open an elevated PowerShell window.

Type the following and press Enter:

Get-AppXPackage -AllUsers | Foreach {Add-AppxPackage -DisableDevelopmentMode -Register "$($_.InstallLocation)\AppXManifest.xml"}

To open an elevated PowerShell prompt, type PowerShell in taskbar Search, and in the result ‘Windows Powershell’ which appears, right-click and select Run as administrator.

Check out this post that follows the same steps.