Start Menu & Button not working in Windows 10
Before you proceed, some of you may want to create a system restore point first, so that you can revert back, if you find that the results are not what you expected.
1] Run the following command in an elevated command prompt, to run the System File Checker.
sfc /scannow
Restart after the scan is over and see if it helped.
2] Repair Windows Image. Open an elevated CMD copy-paste the following and hit Enter
Dism /Online /Cleanup-Image /RestoreHealth
Restart after the scan is over and see if it helped.
3] Make Start full screen and back. Enable Tablet Mode and Start Screen and then go back. See if this toggling has helped.
4] 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.
NOTE: You may do this, to open a Command Prompt or Powershell window. Press Ctrl+Shift+Esc to open the Task Manager. Click on File menu > Run new task. To open a command prompt window, type cmd. To open a PowerShell prompt, type powershell. Remember to check the Create this task with administrative privileges check-box. Then hit Enter.
If the Cortana or Taskbar search is not working, open Task Manager > File menu > Run new task. Type powershell and select the Create this task with administrative privileges check box and click OK.
Let us know if anything worked for you or if you have any suggestions to make for the benefit of others.