Most likely it is blocked. Next time you flush and it fills up try a plunger to push the blockage through.
Depending where your pipes run you can also try tapping the pipe to identify where the blockage is. It will sound hollow after the blockage but full of water before.
I had an a similar issue in a new house I bought and it was actually in the drain outside where the builders has let motar fall in the drain and left it - so you could check that as well.
Phil W, February 2009
