Have you tried flashing with the 'remove customisations' enabled? Not sure that this will help, but worth a try. Avoid 'wipe data'.
If it's hanging at boot after a flash of new rom then it's possibly hardware fault, a device failing, that is causing the freeze. the only way to resolve that is to send it back to Sony.
Was the freeze after a failed flash or mod of some kind?
If not then Sony should fix it under warranty.
Don't know what else to suggest, sorry.
Sony Z1 compact
4.4.4 Kit Slim Evo 8.1 Black
Ext SD: 64Gb