I almost always write with guitar in hand, so my first suggestion when blocked is to pick up the guitar. :)
Assuming a general block (as opposed to a block on a specific song), start by noodling a chord progression. Sing nonsense words at it. Listen to what the mood of the music is trying to tell you. What's the picture in your mind? Can you put words to it?
"Cinnamon Rain" started from the first line of the chorus: "Cold night and cinnamon rain, bringing back a scent I'd forgotten". And I asked why he'd forgotten and got the second line, "Old lights, fire in my brain, bringing back the memory of you". So the memory has been suppressed, possibly against his will. Now how did he get in this situation? And there's a picture of him standing in a great pine forest, somewhere in the mountains, it's cold and just starting to rain, and...
The story will be happy to tell itself to you if you just give it the chance.
Alternatively, there's getting blocked on a particular song. I wrote the chorus for "Half a Chance" at Capricon, the weekend after Columbia fell. And I picked up the guitar and started arguing with what it was trying to tell me, because I'd decided what the story was supposed to be. The guitar had something else in mind. Once I stopped arguing with it and put my preconceived notions aside (possibly for use elsewhere at some other time), I wrote the rest of the song fairly quickly at Minicon.