I can't really help, but have you read the manual carefully. It does'nt like trying to put disks in at the wrong time. The rcd510 mechanism sounds quite clunky to me, not sure how long they last.
if nobody else can help, last resort i'd take it out, take the cover off and have a poke about inside to try and get the mechanism back into sequence. This worked with my sony multi-changer when that had an electrical event and jammed up.
It's like broke anyway, so what harm can you do, you might get lucky if its a mechanical jam up.....