Inspiration is a tricky subject. I myself can only build when I am inspired, this can happen at any time, and well, drawing or scrawling an idea suffices when parts aren't available. I actually prefer to think things out before getting out the pieces, and I can say 'get out the pieces' now since I finally got them all sorted and off the floor.
If you're not inspired to build, then don't, building when you're not inspired drains you to no end. Go find something else to do: Movie? Book? the Interwebz? A walk outside? Any of those can be a nice way to get your mind away from the plastic and if you're lucky you'll find something interesting you want to try building.