Hi, I have had my xps 12 for a few years now and have never had an issue up until the last few months where it did not go to sleep mode when left unattended to and when it did the screen went dark but the computer itself was on. I changed the setting for multimedia use and the problem still persisted. Had to hard reboot a few times due to this issue. I even reinstalled the operating system
The biggest problem due to this is that the screen appears to have been damaged due to the heat given off. The laptop was inside its case while I was out of the house. I picked up the case and it was extremely hot, to the point where, out of fear of damage to the laptop, I held it infront of a fan to cool it down. When I turned it on I noticed a dark patch on the bottom left side of the screen directly over the position of the fan.
The dark patch (overheated LCD) cannot be repaired - the screen needs to be replaced.
If the system is not sleeping correctly, check your event viewer (start-search-eventvwr.msc) to see if there is evidence of a program or driver that's blocking sleep. That is usually the cause. While it's unlikely to have a hardware cause, if it does that's likely a mainboard going bad.
Depending on the system, a screen replacement could run from as little as $150 with labor to several hundred dollars (the latter if it's a touch or convertible screen) -- and if you then find out the mainboard is bad or failing, the system will be a write off, better replaced than repaired.