How do you 'just write' when the words literally will not come out of you?

It's the most common writing advice I see, to just be disciplined and write but I can block out time specifically to write and sit down and just not have anything happen. I don't mean I don't know what to write, I'll have an outline and everything but I just can't make sentences form on the page. It's not even distractions, I can sit there for an hour staring at the screen but still not make myself write. I have no idea how to fix it. Does this happen to anyone else?