"I don't know what to write about!"
In this case, the creative part of your personality is childlike (in a good sense) and is being repressed by your internal editor, which is adult (in a bad sense). So you need to suppress the internal editor to let the creativity loose.
Push away from the keyboard and go do something you enjoyed as a child. Jacks or jump-rope or solitaire. (Play solitaire with a real pack of cards; the solitaire on your computer only aggravates the internal editor, which immediately starts calling you names.) If you decide to do something physical you haven't done in fifteen or twenty years, such as skateboarding, um--be careful.
Then when you go back to writing, write the way you wrote as a child. You wrote stories as a child, remember? How did you do that? In a spiral notebook sitting under a tree? Do that. In a composition book under the bedcovers with the aid of a flashlight? Do that.
Feel silly? Hey, you're a writer! You're allowed to be silly! Once you take hold of and 'own' the concept that you're allowed to be silly, the writing ideas should start to flow again.
"This is junk! Why did I ever think I could write?!"
The second type of writer's block calls for a different approach. (Of course, you can skip rope and sit under a tree if you want to.) In this type of block, you know what you want to say, but when you write it, it's horrible. It's flat, it's awkward, it's just impossible.
The trick here is to write it anyway.
And don't delete it!
Here's where advances in technology have turned out to be counterproductive. We've all seen scenes in movies of the blocked writer. He's sitting at his typewriter in an advanced state of frustration, an overflowing wastebasket testimony to the false starts he's ripped from the typewriter and discarded in disgust.
But remember how that scene is usually resolved; either the writer or his long-suffering wife goes through those crumpled bits of paper, and smooths out one that turns out to be not so bad. And voila, block overcome, and he takes it from there.
Unfortunately, the advent of computers and the delete key have done away with the overflowing wastebasket, and today the writing attempts that are discarded in disgust are gone past retrieval.
And here's something important you need to know. When your writing is going great and perfect words are flowing like water, and also when the writing is going horribly and lousy words are creeping out--both those states of mind are based much more on your mood than on the quality of the words themselves!
So go ahead and write the next bit and keep on writing the lousy, horrible, awful words all the way through. And then save it and print it. Then go do something else for a while. When you go back and reread what you wrote, you will probably discover that it wasn't so awful after all, or you will see how it can be fixed and the right words will come to you to replace the ones that are wrong.