I've only had one case of writer's block in my life but it was a doozy. It came on after I'd sold my first three stories but before any of them appeared in print and it lasted for nine months.
Nine very, very long months. During the time that block lasted, I also turned thirty, lost my job, and got married.
I tried every cure in existence. I tried plotting out stories in great detail ahead of time. I tried free-association. I even went with that old chestnut and had somebody walk in with a gun:
"Holy cow!" somebody exclaimed. "He's got a gun!"
"What do you want?" somebody else asked.
"Don't hurt us! We'll do whatever you want, just don't hurt us."
And so on and on and on, until I finally acknowledged that no plot was congealing out of this mess and tore the sheets in half and dropped them in the trash basket. It was as if all your friends had dropped by and then refused to leave, but kept chattering away and raiding your refrigerator for food. Entertaining, perhaps. But certainly not art.
Nevertheless, every day I sat down to write. Every day I produced great volumes of words. Every day I discarded them all. Until, finally, nine months into the process my hind-brain got the message that refusing to give me ideas wasn't going to get it out of sitting in a chair, writing, every day.
At which point, it gave in and I started writing again.
I haven't been blocked since.
This is the least pleasant cure for writer's block imaginable -- keep on writing and continue to keep on writing until something breaks within you and you can write again. And here's the really scary part of this cure:
It worked for me. But I have no idea if it'll work for you.
Still, when all else fails... what have you got to lose?