The most likely instance of finding me not being able to write is when I am fatigued. When my brain is tired, I just can't push myself to think deeply enough to attempt writing and it prohibits me from writing well. So if I feel I need sleep, I do that then try again. Or I may take a walk or take a shower or go see some sights - just totally disengage for a bit. Usually, after I take some form of break, I find myself feeling guilty that I am not achieving something so I get back at it and usually find the clarity and the energy I need to get at it again - and my work is the better for it.