Calm down, your phone is not dead, you still can revive it simply by flashing a stock firmware using Flashtool. You should be carefully flashing Sony devices when having a locked bootloader.
Download the firmware herehere
Download the Flashtool here and install it.
Go to the folder that contains the Flashtool (Usually C:/Flashtool) and launch it by clicking on either FlashTool.exe or FlashTool64.exe, whether on 32 bit machine or 64 bit)
Return to the folder and open the Drivers folder, install the drivers.(C:/Flashtool/drivers).
On Flashtool, click on the thunder icon, select Flash mode.
Now you should open the firmware select Window. The source folder address should be
C:\User\your_name\.flashtool\firmware .Copy the firmware you've previously downloaded to that folder. IyIt should now appear on the selector window.Now click on flah, plugflash button. Plug the USB cable while holding the volume down button or back button. Drivers should be installed and the flashing process started.
Just wait until the end.
Et voilà.