I assume that you're talking about a window AC unit. You almost certainly have a clogged drain hole.
All AC units condense water on the outside of their coils and they have a provision for draining this water outside the house where it (hopefully) will do no damage.
I don't know where the drain hole(s) is in your sepecific unit. You may have to look inside and clear the hole with a toothpick or coat hanger. There should be drip pan (it will be filled with water) and at the low point of the drip pan there will be a clooged drain hole.
Perhaps someone else here can give you some more specific advice. --Billy