I have been practicing german for two weeks and today, when I entered the app again to practice, it asked me to start again from the basic 1. I still have my Lingots and in the webpage it still shows me the XP graph, but the levels that I completed are gone. What can I do? Thanks!
If you use the process described in the first comment here, does it show the XP and the level you expect?
And yeah, underscoring ConnorHay's suggestion: try logging out and logging back in, or even relaunching your browser or clearing your cache. Sometimes cookies screw everything up.
I'm still confused about the XP. It's great that the levels are as expected, but if the XP (according to the above link) isn't as expected and your tree puts you at Basic I...something's definitely going on. If I were you, I'd write this up and submit via the support link on the left. The Duolingo Twitter account also seems to be more responsive than the support website.
Yesterday I managed to fix it. When I gave up and went ahead to take the placement test, I noticed that it asked me to write the answers in english (I'm a native spanish speaker). To change that, I added the version of german for spanish speakers and suddenly everything was there. I don't know how it got changed, but the good news is that I recovered my levels and XP points :D.