When you say it's come away, can you be more specific?
Each little pane is held in place with putty - black paint mixed in to make it dark - and the lead strip itself which is a wide flat U shape. The glass pieces are pressed into the lead channel and then it is firmly pressed shut with the side of a blade. Over time, the putty often deteriorates and crumbles away. If it is very old or has been exposed to a lot of pollution, the lead can weaken. I also knew an old lady whose free range budgie had nibbled bits out of her leaded windows. It died some time later, whether due to the lead or old age I don't know.
It is generally quite hard to put just one piece back into a window without it looking a bit wonky. If you don't mind it looking a bit wonky you could have a go yourself with a butterknife and some putty.
It might be if you look closely that the whole lot needs re-setting. This is probably best done by a professional, but it basically entails taking out the entire panel and re-setting it in new lead strip (lead 'cames').