Is this a plastic pot or a terracotta one? The way to get plants out of a plastic pot is to turn the pot upside down and tap the pot edge on a hard surface. If it's not a plastic one, then try a hose and wash away the compost in the pot - you may be able to get it out that way and then repot in fresh compost.

Give it a good soaking first; put the pot in a bucket and fill with water to just below the rim. Leave it for 24 hours and then tap it out as described by Spritzhenry. If this doesn't work then take a knife and try to run it round the inside of the pot, tight against the wall. Don't let it get pot bound again!!!!

And if you still can't get it out, get someone on the other end to hold the pot rim, while you pull the other way like crazy...