Send for a plumber.
Or failing that, there might be a 'man-hole' style cover round the back of the house where the plumbing goes into the house. It could be a concrete slab, type affair. If you can get that up there's a good chance that you'll see a blockage. Prod around in there and see if that unblocks things. But be warned. It's (obviously) going to stink, and unblocking it isn't going to be easy or pleasant.
