The pleasure is mine!
I don't think you need to "fear" losing your save. In reality, I imagine your previous save must be somewhere in the system.
But anyway this is out of my control as it's a Windows10 update (I can't imagine any other reason for this problem, it's never happened before).
Example: C:\Users\YOURFOLDER\AppData\Roaming\Stencyl\NEXT JUMP Shmup Tactics\mySave.sol
Or try searching for "MySave" in your folder.
I hope that helps!