This is a job for sub-events! Create your key press condition
On C Pressed. Now right-click on it and add a sub event (I think S is the shortcut). Now add your other conditions
-------Box.Whatever = "closed" | open the box, set box.whatever to "open"
-------Else | close the box, set box.whatever to "closed"
The indentations help your know you're in a sub event. Basically, when you press C, it then triggers evaluating the other conditions. How you were trying before wouldn't work because the "else" of on C pressed is infinite. Sub events fix this by letting you run multiple layers/tiers of conditions off of one action.
I hope that solves it for you!