The symptom of this bug is the mouse and/or touch of your finger work everywhere else but the startup menu.
What to do when the Windows 10 startup menu does not respond to the mouse or touch screen.

Microsoft brought back the good old startup menu for Windows 10, but sometimes it just stops working. It’s nothing you did. It’s a bug in Windows 10.
There is a quick solution. No aggravating, time-consuming reinstall necessary.

Here are the steps to restore the startup menu:

1) Make sure you have saved all your work.

2) Press Ctrl+Alt+Del. Next, click on Task Manager. Scroll to find Windows Startup Application, then right click on Windows Startup Application. Click the End Task option.

3) Now, push the power button to restart the computer.
When you restart, your Windows startup menu will start up!
One customer noted that the Windows photo viewer had also stopped working, displaying an annoying error message. When I restored the startup menu, it worked the same as always.

I recommend that customers who have Windows 7 on their computers stick with it as long as possible!