Hi, Surreal! Here's what's up: If you go outside, directly in front of where the sink is, you'll see a pipe protruding from the wall, approx. 1-1/2" diameter, with a plug in it. Using an adjustable wrench (Channel locks will do in a pinch) remove the plug.
Take a garden hose, and jam it in there about a foot. Turn the hose on, and try to keep the water from escaping (you're goona get wet,

) run it full blast for a minute.
Sometimes this will dislodge the blockage.
If it doesn't, you'll have to rent a "snake", and if that happens, post back, I'll give ya instructions (it's not difficult)