Go into the app, sign into a class (Instructed class or Freestyle ride) There will be a red banner at the top that says "You are disconnected, tap here to reconnect" Tap there and select your bike
If your bike is not there, go to "More" then "Select Equipment" and make sure that you have your equipment added to your account. If Yes, attempt previous step
If no, try uninstalling and reinstalling the app/ check for bike firmware updates in the "More" tab. Try to connect another device.