Everybody has suffered from writer’s block. The most obvious cause is not being sure of what to write. This can be caused by a lack of understanding of the topic, or possibly how to express a viewpoint.
Knowledge. In the case you are not sure what you are writing about you should spend time clarifying your knowledge before returning to writing. Reorganizing the material often helps to create clarity.
Lack of knowledge. Current knowledge is based upon knowledge discovered and used before. This means that no matter how simple something apparently is, it has more layersw of knowledge than could be known by any one person. In the cases where you don’t know everything you should define what you do/don’t know.