After a long useless chat with Fitbit support and then a phone call with Fitbit support, I confirmed and learned two things that may help you.
1) What I first did, plug the tracker into the charger, hold the side button until logo and vibration occur, was the right first step. It’s the equivalent of restarting a phone or computer. And it may have to be done several times before it works. However, I knew this and had done it over and over and this didn’t work for me.
2) Simplified sleep is based on movement while sleep stages is based on heart beat. And in fact, my Fitbit is no longer showing my heartbeat. The green light on the underside is flashing but on the face of the Fitbit nor in the phone app is the BPM working. Therefore, the Fitbit rep concluded the issue was not the app update. It’s that my Fitbit has gone bad for heartbeat and therefore it cannot do sleep stages.
It seems like a HUGE coincidence that the heartbeat stopped tracking at the same time as the update but I do understand that without the tracker monitoring heartbeat it cannot do sleep stages.
