At the back there is a clear plastic chamber - it is probably clogged up.
To clean this, remove it by unclipping the plastic clip (on the black rubber hose) and then the clip on the clear pipe.
Pull the clear pipe upward and then pull the plastic chamber out of the black rubber hose.
Give this a good clean out and replace it making sure that the clips are in the correct position.
This chamber controls the amount of water in the drum... but doesn't allow the controls to know that it's empty properly, if it clogs up.
Good luck.
Robert Welsh, May 2008