from their website
Remote UI
Connect to your Home Assistant instance remotely.
Connection failed: No response from instance
This has happened to me yesterday when I first started using a notebook to access HA while normally I would just use my mobile device. When I finished with the notebook I logged out thinking nothing of it, however it closed the connection to all devices. My only option at that point was to log into HA locally and re-enable the NC by logging into it.
I think NC needs some monitoring capabilities like being able to see the active connections to the HA instance.