Hi, Michele. Is this a white stain or a black stain. A white stain indicates that there is water in the finish but not the wood. Try putting petroleum jelly on the mark and let that sit over night. If that does not work, come back for harsher measures.
A black stain indicates that water has gone into the wood. These are difficult to repair. You will at least have to strip the stained surface, try bleaching with household ammonia, and then refinish. How much work you will have to do depends on the depth of the stain. If you don't have some experience with this, or if it is on a really valuable piece, you should have it done by a refinishing pro.