there are two rods connected to the latch. one from button, one from key barrel.
you have to open the door card and undo the handle, you will find out, what the problem is. small clip missing, which holds the linkage to lock barrel plate.
you may access the rod only by taking the handle off (losening a bit), but carefully, the mentioned plate is cast metal and breaks easily.
you can reconnect the linkage by using a simple hairclip, if you cannot find the original one.
regards, korkut