Whenever I make a level for my game, save it, then close unity, it’s gone the next time I come back. It always seems to do it for the most recent level I’ve made. It takes 15-20 minutes to lay out a full level too, so I would prefer it if my work was actually saved 
Is there anything I can do to make this stop happening?
When you say it’s gone, do you mean it’s not even listed in the project view? Or is it listed in the project view, but empty when you load it?
The level exists as an asset, but none of the objects are in it.
If you place some objects in the scene, save the scene, open a different scene, and then open the first scene again, are the objects gone? Or do they only disappear when you close and re-open the project?
(Maybe someone else will know the answer based on what you’ve said so far, but I’m just trying clarify the situation, just in case.)
They don’t disappear if I do that. I have noticed though that I can seem to avoid the problem if I save the scene, open a different scene, modify something in that scene, save that, then close unity. It’s a bit of a bass ackwards way of doing it, but I don’t lose work.