The simplest way to overcome this is to put your fingers on the keyboard and move them. Write whatever you are thinking. Don’t stop until the screen has 100 words on it. I don’t know why this works, but it does. Maybe it’s the principle of momentum.
I started doing something similar when I get stuck. Just open a blank document and start typing whatever comes to mind. The output is usually nothing I can actually publish (well, except for that one time) but it helps to break whatever is keeping me blocked.