Help!

My Cracklin Rose as lost all it's leaves. I had it outside for the summer, it was doing wonderfully. I brought it inside in well enough time (no frost had touched it). Now, all the leaves have dropped! They started simply dropping! I had the plant under my lamps..and the last leave dropped today. It has enough water, but not too much. So, in my last attempt to save it, I cut off one branch and put it into some water. Does anyone have any idea? Please? I have many begonias, I'm not new to them..and I'm lost at this event. Also, no bugs or anything like that. I was soo healthy... Help! Thank You!