Did you possibly reset the password of the Sync account because you forgot the password? Resetting the password for the Firefox Account because you do not remember the password will remove all data stored on the Sync server because this password is used to encrypt your data before it is uploaded to the Sync server and only this password or a possible recovery key can decrypt this data. Keep in mind that Sync isn't designed to be used as a backup, but merely is meant to sync data between connected devices. Best is always to backup your personal data locally.