There might be more psychological diagnoses, but I've observed two distinct writer's blocks.
Mental writer's block hinders your technical capacity to form sentences, paragraphs, and ideas. It often happens without rhyme or reason (too much fruit for lunch? stubbed your toe this morning? bad talk with your boss?), and settles on everyone occasionally. The best way to overcome it is to write. Move your desk to an empty room up against a white wall, and tape your arms to the desktop so the only thing you can reach is the keyboard (or pen and paper, if that's your style). Write about anything that comes to mind. Your socks. Aunt Mildred's AWFUL brussels sprout soup. Your upstairs neighbor's horse-shoed monster that comes out at night.
After you start, the ideas will flow as you transition to writing for your passion project (if that's Aunt Mildred's soup, give it up now).
Emotional writer's block hinders your soul from creating. It feels empty, like you have nothing to say, even though there's plenty to write about. It doesn't effect the technical aspect of writing; you're still perfectly capable of mastering syntax and grammar (if you could to begin with), but ideas have no value. There's a jumble of concepts waiting to be framed into beauty, but you can't see where the edges fit together and it's too hard to try.
After staring into space and struggling to harness listless mental energy, you close the computer, and think, tomorrow I'll have something to say.
But tomorrow doesn't often bring motivation that didn't come today, and sooner or later the soul bankruptcy from not doing what you love is greater than the emotional weariness and pain that stopped you.
So pick up your pen, choose anything to say, and start again. God didn't give us gifts so we wouldn't use them because it's too hard.