Happens to me all the time. First of all, I don't get upset about it. I can find things to do when I'm not out taking photos, like scanning stacks of old negatives or processing some B&W film that is long past time I did. If that gets old, break out of your rut by taking yourself outside your zone of experience - and I know you haven't been there done that for everything.
Surround yourself with input, something will catch.
Ever been to a roller-derby match? How about professional women's sporting events? Amateur wrestling tournament or martial arts competition? Professional ballroom dancing? How about a kid's birthday party or a social event? All you have to do is advertise on Craig's List or something like that and say you'll be happy to come to the kid's b-day party and take photos, or whatever, for free, and someone will take you up on it. You'll look at a few things differently, you'll find a way to express yourself through the photos you take, and you will have to stretch a bit if you're out of your usual comfort zone.
I like to query Google for local events the following weekend - anything will do. Sometimes it ends up sucking - I went to a "Kiss Exhibition" that turned out to be a bunch of grotty forty-somethings crowded into a tiny room in a hotel selling each other crap. But other times, I've been amazed and delighted, like when I went to an anime festival. Just look for things you haven't done and go there, whether you think you'll like it or not. Think about how you want to approach it photographically and then see if you have the stuff to do it up right, or to do it in a novel way.
That's what works for me, anyway.
Dig this:
http://www.flickr.com/photos/wigwam/sets/
That's how we do it.
Have fun, good luck!
