Bosch WFF 1401 Why has the drum stopped turning?

The problem occurred during the wash cycle of a large curtain, so it may have been too heavy when filled with water. The belt is intact. The drum moves around freely when you turn it manually. During the spin or wash cycle, the water fills and empties as normal and the timer continues to click through its cycle. Is there a sensor that has de-activated the automatic drum rotation? If so, how do I reset it? I would be so grateful for help on this! Many thanks. Fiona