Just tried this…
My USB midi keyboard was plugged in before starting CB but it was turned off until after I opened a project. I then turned on my keyboard while in the project and CB recognized it with no problem.
So, one of my responses is correct…
1 - Keyboard not plugged in prior to starting CB: You need to shut down CB, plug in the keyboard and restart CB (PC only).
2 - Keyboard either on or off and plugged in prior to starting CB: The keyboard should work after you turn it on.
3 - Same as 2 but the USB cable somehow got disconnected after CB was started. Follow the convoluted procedure.
Please let us know if your keyboard was plugged in or not. Thanks.