Hi there, i had the same, phone did not turn on and power led was blinking every 3 secs when i connect charger and didnt turn on :( checked the following:
-pushed mini "hard reset button" beneath the sim card cover didnt work
-pushed vol down and connected usb cable didnt work (should start with android system menu)
Pls do below on your own risk.
-took a heat gun (a hair dryer will work also but more time is consumed). Heat back cover till your palm feels quite hot (but not so hot to prevent touching, dont go that hot). Took a very thin "-" screw driver or guitar pick and opened back cover. Found battery connection and removed connection (not the battery) and waited 10 secs. Replugged battery and connected charger. Phone turned on :) :) :) :) :) pls watch "how to replace sony xperia z ultra battery" video on youtube. It is really easy than expected.
Thanx