The last thing we want is for your work to be lost. You have a couple of options. You can and should save your project to your computer and then upload it to your repository. You can also create an account and save your project there too.
If you are feeling adventurous, you can always upload your project so others can see, remix it, and provide feedback.
Once you save your project, it should be a .sb3 file.
Did it work? Great!