The only thing that has ever worked for me is to get grounded. Writer's block is a kind of panic. Panic is a part of my life. Luckily, this skill is transferable.
I have a few orientation exercises I use for getting back to the present: tapping my knees a few times, alternating left and right, is one. Noticing objects in the room, and coming up with three words to describe each, is another.
Sometimes, there's just something else wrong: I'm in pain, for example, and that is keeping me from getting back into my body, because it hurts in there. So then I have to work on that, instead. I know when I'm starting to feel better, because I'll start writing, automatically.