For Windows O/S: Here is a way to restart a USB midi device that was plugged in before starting Cubase and was running properly but somehow got disconnected during the session.
Once you discover your USB is disconnected and not working do the following procedure:
- Shut off your USB midi device
- Re-plugin your USB midi device
- Turn on your USB midi device
- Go to Devices>Device Setup…>MIDI Port Setup
- Find your USB midi device in the list and uncheck the box “In ALL MIDI” this will make the state “Inactive”
- Recheck the “In ALL MIDI” box to make the state “Active”
- Hit OK
You would think just hitting the “reset” button instead of doing steps 5 & 6 would work but it doesn’t work for me. 
Regardless, you should be all set.
Sincerely,