Users sometimes think they lost all their data and stats when they re-install, update or delete the app for a while and download it back on. That's not the case.
In most cases they accidentally sign up a new account instead of logging into the previous one.
Some things to keep in mind when logging in to QuizUp:
Use "Login Now" if you were using an email based account. Do not use "signup".
Use "Login with with Facebook" if you used Facebook account for the previous QU account.
Use "Login in with Google" if you used Google+ account for the previous QU account.
Facebook connect: Must be logged into your FB account on the device FB app.
Google+ connect: Must be logged into your Google+ or Gmail on your device.