Overthinking or over analyzing what you have already written down, fear of not doing as well or better than you have written in the past, fear of how others will judge your work, substance abuse and mental health challenges, attention deficit, distractions in your work environment such as noise, clutter, bright lights and so forth, lack of rest, food or exercise could do it too. Parts of the brain responsible for creativity can sometimes be fatigued and underactive.
The cause of writers blocks can many different things, from anxiety, depression, pressure, a sense of failing those are you. This causes your brain to go into over time and you begin to over think everything you are doing.
