unable to log on to app?
Is the network down? I can log into a computer, but not my mobile device?
What kind of result are you getting? This is a good introduction to how to report bugs and errors in a way that is as helpful as possible.
Not knowing anything else, here's my generic app troubleshooting flow that should be applicable to any smart device:
0 Make sure that you're using the same login info for the website and the app: It can be surprisingly easy to create a new account and not realize it.
1 Check your network connection--yours, not the site. Turn off the wifi and turn it back on again, try your cell network (if applicable or possible), try a different wifi network if possible, etc.
2 Restart your device.
3 Check for updates for Duolingo.
4 Restart device after updating.
5 Reinstall the app.