Users have reported that MotiFIT Run and MotiFIT Ride occasionally get stuck on a 'Waiting for GPS' prompt, disabling the START button. We are looking into permanent solutions, but in the meantime the recommended course of action is to force stop and restart the app.
To force stop an app on Apple Watch:
Press and hold the side button for five seconds, release, then press and hold the Digital Crown for five additional seconds. Then restart the app as usual.
(updated: April 12, 2017)