Trying to turn on the Sony Xperia Z5 and instead being left staring at a blank screen is mighty annoying, but thankfully Sony suggests some fixes for this particular Z5 problem.
One potential solution is to try a forced shutdown. Open up the cover for the nano-SIM card and memory card slot, and using the tip of a pen or a similarly-sized object, press and hold the OFF button for a couple of seconds. The Sony Xperia Z5 will vibrate three times, after which you can release the OFF button. Try turning the device on to see if the problem is solved.
