This may come about due to a worn or loose USB cable or human error. Unlike Schwartzpeter, you can disconnect/reconnect (hot) your hardware for White Cat without having the system crash. You do not need to restart White Cat to reset the connection.
Hot plugging your DMX interface:
If you use the Double DMX outlet (Enttec Pro + ArtNet) to control the video:
If this doesn't Work (Still no green LED):
Sometimes with Windows, connecting midi can make it jump or skip. This problem is usually apparent on machines that have not been updated and disappears after the update. However, tired and worn USB ports can also cause a disconnection.
When you move your midi controls, if nothing changes in the feedback, no info type Chan: 0 Pitch: 56 Vel 127, If this happens during a performance, do not panic!
Midi ports to the left (in orange) are MIDI inputs (signals coming to White Cat). Ports on the right in red are the midi outputs (White Cat sending to the midi interface).
If it is still not working: