[Q] Phone doesn't do anything, only backlight is on topic

Hi. My friend has an SP, it's stock and unrooted, and a few weeks ago the phone started to hang on black screen with backlight on. Of course he tried to reset the phone with the button under the battery, with no success. He returned it to the store, they only reflashed the stock rom, it worked for a day now it's the same. Has anybody find any solution to this? I tried to search for exact similar problems, but I couldn't find any with proper answers. I think he is on 4.3. Thanks for any help!