My wife's L925 (which is running Black) with DP8.1 does something similar to this sometimes. With her phone, its just a black screen and cannot be woken up or switched on or off. The only way to get it back up and running is to hold down and power for about 15 seconds, it then reboots OK , but the time and date are screwed up, I believe it goes to the same date and time every time this happens, from memory I think it always defaults to around 3.10am and a date from about 8 months ago. After rebooting using the above method and then resetting it to the correct date/time, it will then work fine for anything from a few days to a few weeks. Next time it does it, I'll look at the lockscreen settings and disable the calendar, unless anyone has a better idea.