Make sure you create a backup every now and then. Especially if you just finished a new song. The browser shouldn't delete your data, especially if you installed the app, but there is always a chance.

If you have a backup, you can import it here, they will be added to your existing data. (if you already have the same song, a duplicate will be created).

If you want, you can also delete all your data here, once deleted it can't be recovered. Don't worry you will be asked to confirm before anything is deleted.