If you can't understand anything at all, it may be because of a mental block. It is important to remember that your natural human intellect should be able to comprehend the majority of topics if you are given enough time and a positive mindset towards understanding the topic. Try to write down the parts of the topic that you do understand as this will help to unlock your brain.
Aphasia is a condition that can affect your ability to read, write, or say what you want to say. Some people with aphasia have expressive aphasia (problems saying what they think), while others have receptive aphasia (problems understanding what others say). Most of the time, aphasia occurs as a result of damage to the part of the brain that controls language.