Near the End

I have encountered a problem within the game. I am at the point that Simon and taken the cards and tied up my friends.

I found the knife and sharpen it and used it to cut the ropes, I had to use it twice to cut the ropes and now, the knife has disappeared and I can't cut the vines to get the second key.

Any recommendations?

I am having the same problem, it says that there is nothing to do in any of places, and have nothing to cut the key free now.