If your sink or bath isn’t draining properly, the chances are that you’ve got a blocked pipe.
Minor blocks can be removed fairly easily using a chemical drain unblocker. If this doesn’t solve the problem it could be worth removing and clearing out your sink’s u-bend (or p-trap) if you think the blockage might be there.
For major blockages or blockages you can’t locate, it’s best to call out a professional plumber. They’ll be able to find the problem and have your pipes back up and running quickly.