It is possible that the software on the LabQuest that supports the LabQuest Viewer functionality can get into a bad state. To recover from this issue, reset the LabQuest software by doing the following:

LabQuest 2 or LabQuest 3
1. Launch the Connections App.
LabQuest 2—Tap the Wi-Fi icon
LabQuest 3—Tap the Connections App from the Settings Screen
2. Tap the icon* associated with Viewer.
3. Tap Off and then tap OK.
4. Again tap the icon associated with Viewer.
5. Tap On and then tap OK to restart this software.

Original LabQuest
1. Tap Home > Control Panel > Viewer Setup
3. Tap Off and then close the app.
4. Again tap Home > Control Panel > Viewer Setup
5. Tap On and then close the app to restart this software.

If this fails to resolve the issue, try connecting your devices to a different network.

* Why are there no icons/gears on the LabQuest Connections App screen?