I'm currently in a very similar situation.
I spent most of my late teens and early twenties writing prolifically (350+ songs) in all kinds of genres, simply for the joy of it. Then, somewhere about a year ago my entire brain just seemed to dry up. Everything was too obvious, too derivative, too this, and too that. It had all been done, and better.
Haven't quite managed to kick myself out of it. This may be a cop-out, but I think it's a consequence of the tendency to be over-analytical. To obsess is to eventually come to know (or think you know) everything about whatever it is you're obsessed with. Couple that with a desire for individuality and originality, and every time you sit down to do something you find your ears filling up with clichés and references, eventually driving you nutz.
I've stopped trying to make music (actively trying only makes things worse) and have been taking some time away to work on other things (sound design for one) until I can find the joy again. Hopefully that'll be soon. But if it never comes back that's ok. It was something I did, and did well, and now I'm just interested in other things.
@georgi.m, I think the radio people want that to be true. It makes people easier to categorize and market too. But over the last 10 years I've moved from "alternative", to math-rock, to counry, to metal, to canadian indie-folk-pop, to electronica, to IDM, to ambient, which is where I currently find myself. Though I can feel it starting to slide into neo-classical, which I'm sure will eventually turn into classical proper.
To live is to grow, no matter what the advertisers tell you!