You do not say if the stain was on the wood or what. If it is still on the wood after you have stripped it back to the natural wood, & if there is a Dark stain; then buy some Oxalic Acid Wood bleach (Home Depot, etc etc) and, this is important - with Gloves etc on to protect yourself, make a paste with some oxalic acid and a little bit of water and apply it to the stain and leave on for several hours or overnight. Remove it carefully and with a rag dampened with Borax in solution, wipe away any remaining Oxalic Acid from area, then pat dry. Obviously the wood should be wetted as little as possible - hence the "Paste" . This got rid of a black stain on a hardwood floor for me which was there even after heavy machine sanding.
Souzana Who got this advice from another helpful interneter, John.