I have been known to unwind a wire coathanger and simply insert it into the U bend, and continue to push until the blockage either clears completely or dissipates enough to allow the water to travel more freely. Once the water is moving, the rest of the blockage generally can clear itself.
If it is just tissue, then you are in luck; this will dissolve and disappear quicker than, shall we say, other substances that may also be down there.