I don't know what to do ? Yesterday it was working perfectly and now it doesn't recognise anymore skyrim !
Any help ?

If you normally start the game using SKSE then run the Skyrim launcher (no need to start the game). Boss should then detect Skyrim.

I can't launch the Skyrim Launcher, because there is the huge update (4,7GB) downloading and Steam won't let me launch, my connection is very slow, is there an other way to launch Boss or to cancel this update ?