Sometimes just blowing into your hose connected to the mash tun is enough to get the mash unstuck. (When brewing on a larger scale, you can inject it with compressed air.)
Your next option, should that not fix things, would be to increase the temperature of the mash. Since most homebrewers mash in plastic coolers, directly adding heat often isn't an option, but if you've reached starch conversion and don't mind disabling your enzymes, you can hot water - as hot as boiling. Be careful though: If you add too much water, then the mash will be diluted and you won't be able to sparge and still make your target gravity.
Still not flowing? You can try stirring the hot water in to circulate it, but keep in mind you'll have to recirculate the wort for a while after you stop stirring so that you'll have a good grain bed that filters out particles. As you stir, make sure to scrape the manifold to clear any grain that might be clogging it.
If it flows again, make sure to keep your drain/sparge rates slow so that the grain bed isn't disturbed.
When all else fails, you may have to pour your mash through a strainer or colander to extract the wort. Try to minimize splashing, as hot side oxidation can occur.
It's easier to take steps to prevent a stuck sparge than it is to try to unstick one, and there are plenty of steps that can help avoid one, but that's a whole new question.