There is a product called 'Pressure Plumber' made by Mer (if you google them you can buy it online) you place the aerosol over the plughole and give it one push - there is a shot of gas like substance, which pushes anything that is lurking in the u bend through - you will be surprised what comes out in your drain.
I originally bought mine from QVC the shopping channel, but I don't think they sell it anymore - I usually buy my refills from Mer online.
You could also try soda crystals and hot water, also effective with plughole smells.
Unsecrewing the u bend, as mentioned by two other people is a good idea aswell.