It could be a worn belt has finally given up the ghost. Theoretically that should be replaceable, but these mechanisms are complicated and not easy to fix. Alternatively, if you disassemble it sufficiently to see the mechanism you may be able to locate a catch or lever which is jammed and needs to be released on order for the heads to retract. Does the Stop button do anything? You might find it's jammed and all you need to do is work out what is jamming it.