When it happened with my kitchen sink, I moved everything out of the cupboard and took the piping apart. Turned out the sink unblocker had not worked because my son had cooked himself some Bird's Eye burgers and poured the fat down the sink and it had set solid the the pipe ... a perfect round. I was very pleased with myself until I had to put pipework back together and could not fathom it out. I knocked the door of the builder next door hoping he would do it for me (even risking hearing the latest in his 4 year divorce battle), such was my desperation.
He was out so I went back to the kitchen floor and carried on swearing til the air was so blue I had to open a window. Eventually I did. Daft thing is, had my son been in, he would have figured it out straightaway.
N.B. I now make my own virtually fat-free burgers.