In my regular job, I teach a wide range of students who run the gamut from fourth grade to adulthood. Much of what I do involves teaching writing, and it's common to hear some form of the complaint that I can't think of anything to write. While it's tempting to say that there's a single magic formula to cure all writing problems, the sad fact is that no universal solution to the problem of stuckness exists.
My own in-class approach is to get students talking about topics that interest them, or to get them thinking along lines they might not have explored. But one of the most frequent pieces of advice I give is to write the way you talk. I don't give this advice because I think my students' prose should sound as ungrammatical as their everyday speech ("I think I did good on my test yesterday" often makes me cringe), but because it's a way of becoming unstuck. Talking something out is often a great strategy for idea-generation, or for fleshing out previously-generated ideas. I've told some students that, if they have voice recorders, they should try talking to themselves and listening to the recitation of their own ideas. A lot of it will be lame, but buried among the bad ideas will be several good ones, and that's all a person needs, really, to start writing.
As Robert Pirsig notes in his classic Zen and the Art of Motorcycle Maintenance, no one can remain stuck forever. Stuckness is best thought of as a starting point, a blank slate, a field of potential. Stare at the empty page long enough, and your mind will begin to move of its own accord. Just let it happen peristaltically. And try talking-- to yourself or to others-- as a way to generate ideas.