Blocked

Most of us tend to experience an occasional inability to form words.
This experience is affectionately known as writer's block.

In my experience, the primary cause is associated with focus.

The most common deals with a distracted focus.
This kind of focus is usually lost due to other things.
This often happens when one is expected to write and finds other things more interesting.
It can also happen when your mind is focused on things other than your writing topic.
Incidentally, it is also why, under pressure, this form of focus-loss often dissipates.

The second most common cause is that of the absense of focus.
This kind of focus is usually lost due to lack of discipline.
This often happens when one lacks a definitive timeline.
This form of focus-loss can often be resolved through writing more-often.

Note:
One common way to recapture the ability to simply write down everything.
Called "stream-of-consciousness", the goal is to allow your mind's tongue an outlet.
After a small season, the mind will often return back to the desired focus.
It will, of course, require some degree of editing.