This has happened several times in the past few years and is usually associated with an update. So, once again, the app tells me there's a new update and my One won't sync. Neither will the new update install. Resolution in the past has been done through the site, nothing on my end. Will this resolve itself once again?
I understand the pain, personally, I think they are just trying to force people to upgrade devices. II even tried adding my One back on using the MS App on my tablet and it will not find the One, even when it is connected to the tablet for charging. Dongle also cannot find the One. Fully charged in all attempts to reinstall after update caused it to stop syncing.
I had the same problem - I followed these steps to restart my One and it synced: https://community.fitbit.com/t5/One/How-do-I-restart-my-Fitbit-One/m-p/1295584#M56557
Hope that helps you, too!
Best AnswerThis process worked for me too! Mine stopped syncing 10/24 and all my data is now updated.