What do you do when the Start button stops working on Windows 10? Here is the solution from here

1. Launch Task manager
Press the [Ctrl] + [Alt] + [Del] keys on your keyboard at the same time – alternatively, right click the Taskbar. Then select Task manager.

2. Run a new Windows task
When the Task manager window opens, click the More details option to expand it, then select Run new task from the File menu.

3. Run Windows PowerShell
When the Run new task dialog box opens, type powershell, tick the box for Create this task with administrative privileges and click OK.

4a. Run the System File Checker
Type sfc /scannow into the window and press the [Return] key.

4b. Reinstall Windows apps
Launch the Task manager and open a new PowerShell window with administrative privileges, as explained above. When the Windows PowerShell window opens copy the line below and paste it into the PowerShell window by simply right-clicking at the blinking PS C:\WINDOWS\system32> prompt, or by pressing [Ctrl] + [V] on the keyboard:

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

Wait until the app download and installation process completes — ignore any red text that appears — then restart Windows.