If your Vent loses connection with the Hub there are a few steps you can try. Continue down this list until the connection is restored:
Power cycle the vent by removing and reinserting the battery pack.
If the vent LED continues flashing after the power cycle, this means that it has accidentally been reset and you will need to delete that vent from the app and add it to the room again.
If you did not see the light flash, we'd recommend removing the battery pack and trying a new set of batteries
If you are still having trouble after 30 minutes Power cycle the Sensor.