I went riding on a very very muddy mx track last week and my back brakes make a loud grinding type noise now. When i press the brake, i can feel the pads rubbing on the disc. I have to lock up the tires to have any brakes at all. Any idea of what can be wrong? Thanks, Ant.

take the rear caliper off and see if anything is wedged in there.

Check the pads, they might be worn out.