The gearbox has failed
Remove door handles, spindle and Euro securing screw
Remove the mechanism by unscrewing ~12 screws – warning, these are the ones which go into the door itself, should be ~25 – 30mm long and look like wood screws. If the threads are metric like then put them back as they will be either securing the gearbox to the mechanism OR various parts of the mechanism to itself
Pull the mechanism from the door, you may need to gently lever this out taking careful note of how the mechanism is constructed particularly if you have top and bottom bolts, may be helpful to mark where they dovetail into the mechanism with nail varnish – pink looks lovely 
Examine the gearbox for obvious mechanical failure particularly the spindle follower (square section where the spindle goes through) and rear of the gearbox as bits sometimes poke out
Gearboxes can be jack-in-the boxes so I don't open them, we do not replace broken parts but if you are on this forum, what do you have to lose, it's b0rked anyway
If you need a replacement, send me a photo – I'll be cheaper than any locksmith