You don't know
Your brain cannot recall something you never seen or know about at all. To resolve this, read books, learn from people or search the internet.
You can't recall
This is something you know or learn before, but your mind cannot recall it anymore. If you have this kind of problem too often, it means you do not use what you learn often enough.
You can't apply what you know
Your understanding of what you know is still raw.
Method 1: Look for examples of how the thing is being applied successfully. Pay attention to the key steps in the problem solving process and commit them to memory. Next try to remove the solution and repeat on a blank piece of paper the solution steps, notice where you are stuck and go back to the example to intensify your understanding of that step again.
Method 2: Write down everything you know about the situation and all the ideas that come to your mind for understanding or solving the problem, never mind if they are right or wrong yet.
Next, review all your ideas, keep those that are accurate and make sense, refine those that are inaccurate, spend longer time thinking on those points that you are not so sure until it becomes as clear and obvious to you as 1+1=2.
Imagine you are in a maze and lost, but everything that you need to know is in the maze, you are going to walk around it and make markings that you can recognize, you must be systematic in the marking you draw and be patient and sure in every move in order not to mess up your own line of thinking.
But even when you feel messed up you just need to clear your mind and begin your thinking right over again. Every time you do this, you should gain a better picture than before until you are as clear of your solution as you see something on your hand.
Work and break for Inspiration
Do you remember a situation where you try to recall someone's name but just can't. However, when you give up and go about doing other things, the person's name suddenly pop into your mind?
There is a part of the brain still working on the question you gave to it while you apply your consciousness to other activities.
This is the magical power of our brain. The brain can search for information that exist in your brain, those things you have experienced before; It also has the power to synthesize existing information to come up with good guesses and new ideas but it cannot search for information that it has never experienced before.
Therefore it is important to increase our exploration as well as our powers of memory and analysis. A character that is determined, patient and persistent also helps because these qualities help us stay on the problem for as long as neccessary; collecting information, paying attention to inspiration, writing them down and acting them out to get further feedbacks until the solution is found.