I've got a couple of tricks.
Number one: If it's a multi chaptered piece, I reread the beginning, trying to come at it as if I'm just a reader instead of the author. If it's posted online, I read it online instead of from my own files. Or read some other piece that has some of the same themes, my own, or someone else's. Usually that jump starts me.
Number two: Make myself grind out just a couple more hundred words from wherever I left off. Yeah, I know it's crap. I'll probably come back and delete it afterwards. But just write it, damn it! At least I know where I don't want to go.
Number three: As a routine, I usually have at least two stories in progress at any one time. The stories are usually very different. One may be very dark and violent, the other is funny or a simple romance/adventure, or just a lemon fest. The point is that it's different. If I get stuck on one, I switch to the other and vice versa. More than three is usually too much for me though. Something will get lost in the shuffle.
Number four: Take a break. Pick up a hobby I've put on hold for my writing or catch up on those books and movies I put off.
Number five: Put myself on the spot. I usually start to fade out at about the 3/4 mark on a chaptered story. If I haven't started posting the story yet, I do it now. Reader responses often inspire me. And once I've posted it, I feel obligated to finish it in a reasonable length of time.
None of these work every time, but usually one of them will work. Muses can be fickle...Don't let them hold your work hostage.