Here are the instructions:
1. First, restart the computer
2. Once you see the logon screen, do not select your user name or type anything, but look in the bottom right corner of the logon screen. You should see 3 icons. If you don't see any icons, then just click anywhere on the logon picture and then check again
3. Click on the power icon and then press and hold down the Shift key, and while still holding, click on Restart
4. Continue to hold down the Shift key until you see the "Choose an option" screen
5. On that screen, click on "Troubleshoot"
6. On the next screen, click on "Advanced options"
7. Next, lick on "Startup Settings"
8. Finally, click on the "Restart" button
Your computer will restart and offer you several choices. Press 5 to enter Safe Mode with Networking. You will see the black background with no colors. Also, many apps won't work there. Don't worry - it is only temporary. You just need to right-click on desktop and select Refresh. After that, just restart the computer the usual way.
After the restart, you Windows Start button should fully work.
Let me know if this helped.