Milly01!
I have seen this before but not in a while. In the cases where this happens the name of the level is different than the name of the package and it causes your level to show up in the content browser with a different name than the file.
This has been known to happen when renaming levels on disk instead of in the content browser. Renaming levels in the content browser is the preferred way to rename all assets including levels as there can be side effects otherwise.
There may be other ways that your content got into this state, but lets see if we can get it back into a normal state.
Try this:
In the content browser, select all the landscape materials and right click on them (this will load the level packages).
Click on some other folder in the tree view then back to the folder containing your levels (this will refresh the view).
See if your levels now appear. If they do, select them all and resave them then restart the editor. They should show up normally after that.