I have a mid 2015 15” MBP also and within the past few months it will not wake from sleep if left more than an hour. The screen just goes black. I have had this mbp since Dec. 2015 and it didnt use to have this issue. I have Apple Care and they were no help on the phone. I dont understand if you are saying you cant shut it down or not or if its shutting down by itself. I shut mine down now anytime I am going to be away from it more than an hour.