Saved classes disappear from model + how to delete

I followed the youtube tutorial ( and created a music ontology. I saved it. I exited the browswer. The next day, the ontology is still there, but all the classes I added have disappeared. What should I do? (I understand that there are other tutorials as well, but when information I entered disappears like this, I'd like to understand what I did wrong).

I also do not understand how I delete a class that I have added in the visual UI (I'm sure I could do it wth sparql once I figure out how to run sparql queries, but it seems weird that I can't do it in the visual UI), or how to change the api. Screenshot by Lightshot