Yesterday I installed the app "Image Converter" to convert png images to jpg. After having successfully converted 5 images, a pop up appeared in the app asking me to rate the app. When I pressed cancel, my tablet froze. A first.
Not even staying on the power button for at least 20 seconds helped. To figure out how to do a hard reboot I had to fiddle around a bit. Lucky I immediately found the combo on the first try.
Press the POWER + Volume Up buttons together for around 5 seconds. The tablet will then reboot.
Update March 21 2018: This also applies to the Sony Xperia Z2 phone, probably also to other Sony Xperia devices.