Level got deleted

Hi everyone, today one of levels has just disappeared :C
It was the main part of Level 1 in World 3..
I started Mario Builder and loaded Slot 1, where my game is. When I opened the level manager and loaded my Level 1-A in my third world, I saw an empty level with the standard background and music set. My level is gone
And it is the only level thats gone.

It took me about 1 & 1/2 hours to make and now I have to make it again.. I don't know if I can make it as good as it was before.

Anyone know how this happens and what to do to prevent this from happening?

Well I can only think of 2 things how it could have happened, 1st someone could have set your course template into blank course, or 2nd you exited to the main menu before saving, because the game doesn't autosave levels if you leave to the main menu. I don't know any other reasons, sorry for what happened.

But I know that I have saved it and nobody else is using my Computer. Oh well, this will not prevent me from finishing my game though. I just leave this level in the level manager empty and make a new level for the first level in my world 3... I don't know what caused this but I will be more attentive from now on.

Pause creating your game frequently to test it and save a copy after that if all your game works correctly! To save your backup just copy your slot to the separate folder which is not readable by Mario Builder. This will avoid more loss which you even may not notice.
It is even recommendable by me to save backup also to your flash drive to avoid some kind of problems when some kind of bad happens to your computer for example and to your online account to avoid some kind of problems when you also lose your flash drive.