Some days, I just don't feel like writing. As much as I love it, I just have other things on my mind or I'm just not motivated on those days. I know a lot of writers who have the same problem.
I think the first time I really encountered this head on was in high school, in my first creative writing workshop. It was an hour and a half long class and my teacher insisted that we spend the first forty-five minutes writing silently in our notebooks. She would write a prompt on the board and we were free to respond to it or write what we wanted.
Most days this exercise was fun for me. I got to do my favorite activity uninterrupted in the middle of the school day. More often than not though, at least one person in the class would say, "I just don't feel like doing this today." Our teacher then would gently remind us why we had to do it:
If you're going to write, the first step is putting pen to paper. Just get started and see what happens.
I doubt that every person taking that class wanted (or still wants) to be a writer in their adult life, but for me, that advice has been incredibly valuable. Even on days when I don't feel like writing, I still try to put pen to paper. I try to write at least a few words, because after those few words (most days) I don't stop. If I can just get a sentence or two down, usually that's enough to get my thoughts flowing.
This same creative writing teacher would also answer complaints of "I can't think of anything to start with" by saying, "Write any words that come to mind. They don't have to make sense. They can be nonsense words. Just write anything until you get past the creative block in your head."
And it works. Write any words that come to mind. Anything. See what happens. Just get started.
Good luck, writers.