Your app's settings menu will indicate that your Hub is disconnected if the Hub has fallen offline. When this happens, you may notice that your Hub's indicator light is a solid blue and it's only offering simple challenges to your pup (this is the Hub's "offline mode"). You may also notice the light blinking green as your Hub attempts to connect to its remembered WiFi credentials.
There are a few reasons that this may happen:
The Hub may be having trouble picking up a signal. Try unplugging your Hub and plugging it back in, unplugging your router and plugging it back in, or moving your Hub closer to your router,
If you got a new router or changed your WiFi name or password, you will need to go through setup again to connect your Hub to the new WiFi credentials. Click here for setup instructions.