Make sure you see the flashing blue light on the console or handlebar post of the bike. (The flashing indicates that the bike is in Bluetooth pairing mode). If you don't see the flashing blue light, then try to unplug and re-plug the bike so that the Bluetooth mechanism resets and the light flashes.
If on any bike other than a bike with the mounted tablet, go to a class, and tap the banner that says DISCONNECTED at the top of the screen. Your bike should show up in the devices list that pops up.
If this is not the case, then try to use the app on another device to see if the issue persists. If it does, then the culprit is most likely the Bluetooth mechanism in the bike. If it is an EX-1 or EX-3, then the console would need to be replaced, if it is any other bike, then the bike's Bluetooth mechanism would need to be replaced. If it doesn't, then the culprit is most likely the original device. If that original device is the mounted tablet, then the mounted tablet would need to be replaced.