Well the fact that you managed to ask this question shows me it's not a big problem :L
Sometimes we just blank, perhaps you're tired, low on blood-sugar levels, stressed.
The interesting thing about creativity, and so the questions you might be referring to, is that they usually come in idle thought, when we are relaxed and not actively thinking about them.
It's hard to remember something by actively forcing yourself to remember it, in fact, studies have shown that stressing over remembering something is the first step in forgetting it. They suggest to forget about the thing you're trying to remember, and if you really knew it, it will come to you passively later.
People around me always say "forget it, it'll come later" and it does haha.
Hope this helps, good question :)