Driving your motorcycle into a brick wall at high speeds (AKA Writer's Block)

There's a big difference between not knowing what to write and not writing. For example, sometimes you're struggling to keep up with school and writing, so you shelf writing for two weeks while you descend into the utter misery and horror of exams. Or, something's happened - a breakup, you've moved, a family member's sick, the Death Star has blown up your planet, whatever - and you just can't write because you're busy running and not thinking straight. These are all perfectly awesome reasons to not write. 

But what happens when you sit down to write and you can't? You have no clue what to write? You sit down and your brain goes fuzzy and your fingers are stiff and your characters are flat? That's called writer's block, and it sucks. (It feels like not even the Avengers can save you. Yes, it's that bad.) It kinda feels like driving your motorcycle at high speeds into a brick wall. (Not that I've ever experienced this, but still. I'm a writer. I have a good imagination). 

Ok, it's not a brick wall, but I'm sure it'd still hurt. 

In order to cure the world of this terrible, mind-numbing disease, I bring you, The List of Amazingness that will Surely Cure your Terrible, Mind-Numbing Writer's Block for (Possibly) Ever. For short, TLoAtwSCyTMNWBf(P)E. 

-Walk away. Give it a break. You've earned it. Writing is hard, and no one can stare at a screen forever. Give it at least a couple of days, possibly a week. 
-Eat well. You can't write if your brain is mush. 
-Get some exercise. See above. 
-Talk to people. Real people, not your characters. Yes, such things exist. 
-Read a book. Watch a movie. Recharge your creative juices. 
-Go for a drive. Or a long walk if you can't drive. Think a lot. Take pictures. Do a cartwheel. 
-Sit down with your pet/baby sibling/stuffed animal/favourite plant and talk to them. (It's probably best to do this in private so you don't get shipped off to an insane asylum.) I've found one of the best ways to figure something out, whether it be a plot issue or a flat character, is to just talk out loud. It's awesome to have someone to discuss it with, but not everyone has someone that amazing. So just talk to someone/thing who can't talk back, and don't stop. 
-Sit down and force yourself to write. It will be hard. It will suck. But just force yourself, no matter how painful. 

If all else fails, throw something random at your novel and run with it. It doesn't have to make the final draft, but it does have to get you writing. I've included some (quite random) ideas below. 

-Aliens from the third moon of Jupiter invade and capture your love interest. 
-Someone discovers an old time machine hidden under the floorboards. 
-A guy with a gun walks into your main character's school. 
-The main character's (MC) best friend lost your MC's iPod... was it on purpose? What it is accident? Who's paying for it? 
-Random explosions. 
-Random gunfights. 
-Random car chases. 
-Actually, anything random that you'd find in an action movie would work well. Or you could do a Mission Impossible and throw someone out of a building. 
-Something happens to the parents. Someone gets in a car accident, they get a divorce, someone's in a mid-life crisis, whatever.
-MC's crush was single... but not anymore. 
-The villain of the novel went to the same school as the MC, and the MC accidentally hurt the villain, hence why the villain hates the MC but the MC doesn't remember it. 
-There's an old guy with a long beard and a chainsaw following your MC. 
-Someone knocks your MC on the head and they forget everything. 
-A T-Rex appears at the high school and starts snacking on students. 
-Your MC's favourite teacher falls ill and a new teacher comes to school. Mean? Nice? Totally insane? All of the above?

Obviously, these ideas are a little out there, but you get the idea. Do something crazy. Do something stupid. And write. 

Always write.