All depends where the blockage is. First try a good toilet plunger not a sink one !!! or try using a mop head vigoursley up and down if you havent got a plunger. Or bail it out if it is overflowing and then poor a bucket full of water down it from a height. If that fails you may have to open up the soil pipe rodding eye if there is one and either rod it out or jet it out. Good luck.