My wife's z3 compact phone suddenly stopped working without any incident to cause it. When trying to power it on the white screen comes on with "Sony" on it and flashes for about 2 seconds on, then 2 seconds off. Holding the power button and volume gives haptic feedback but has no effect. Anyone have any ideas what to do ?
I've seen that before and it was hardware related, you could repair the phone with XC, that could solve issue, if it doesn't, you have 2 options, either contact your Local support to get it inspected or you could DIY but it'll probably cost you the same as a new phone.
There are two more things that you can try before you send the phone to a service center as @uliwooly suggested.
Firstly i would suggest that you try removing the SIM card, and then try restarting the phone. If the phone restarts successfully after you remove the SIM card it might be the SIM card that needs to be replaced. Secondly you could also try to remove the SD Card if you have one. If the phone restarts successfully after you remove the SD card, your SD card may be damaged and in need of replacement.
If none of these measures manages to get you past the boot screen I recommend that you try to perform a software repair as previously mentioned by @uliwooly. If the software repair doesn't help, contact your local support team in order to have the phone sent in to an authorized service center. If you have any questions regarding the service, such as routines, warranty and consumer laws, these type of questions are best answered by your local support team as well, as they vary between markets.