I just updated Snap 5.0 and installed Sen2Cor last night. I did a South session with a S1A scene (my study region is in the middle of two scenes), and luckily, I saved and exported all of the steps, because though I saved the session, I could not reopen it today. This evening, just now, I repeated the work for my Northern site, and saved the session after exporting all the TIFs, and sure enough, the same thing occurred. 
So, people, please save all your products and export your layers.
Yes, the same happened to me, days of work are lost!!!
@ESA: is there a fix to such a situation?
It is not possible to investigate something that has not been thoroughly described, and is therefore unreproducible.