Sony Xperia Z4 (known as Sony Xperia Z3+ in Europe). Smartphone with 5.20-inch 1080x1920 display powered by 1.5GHz processor alongside 3GB RAM and 20.7-megapixel rear camera. It was released in 2014.
If holding the power button for 30 seconds or more does not work, I recommend letting the battery completely drain. Then charge it to see if it turns back on. Your logic board may have gone bad.