Everyone has experienced a mental block when trying to remember something. This happens most often when you are under pressure to remember, such as during a test.
Instead of trying to think harder, it’s often helpful to stop trying to remember and move on to another problem. Frequently, you will find that just giving up the search will be enough to remove the block and the fact will instantly come into your head. This is because you are more relaxed. It’s also likely that your subconscious will continue to think about the problem and the answer will pop into your head a few minutes later.