Drum won't move after belt came off Zanussi FS 1555 W model?

My Zanussi washing machine is just over a year old and the guarantee ran out in August.

Got home after a 13 hour night shift (the night the clocks went back) and inadvertently overloaded the machine with towels (forgot the get heavy when wet).

The machine stopped and the end light started flashing and machine made a quiet beeping alarm noise. The manual doesn't tell you what this means. I took the top off the machine and found the belt had come off. The belt is as new but when I tried to put it back on I found that the drum won't turn at all.

Do you know if there is a safety braking system which has locked the drum or similar? I'm missing something here and hoping an expert can help. Hoping and praying its something simple as the machine isn't very old.

Please help asap.

Many thanks