Did you recently flash a custom ROM? If so, did it boot before this issue? When you press and hold the power button, does the phone vibrate? How did you try to booting into flashmode and fastboot? As a reminder, for flashmode, press and hold the down volume key and plug in the cable, for fastboot press and hold up volume key. Try both of these while connected to your computer with Flashtool open, and check if you've succeeded. If yes, go ahead and flash your stock ROM, problem solved.
If you didn't recently flash a ROM, or none of the steps detailed above are working, it is probably a battery issue, or a kernel issue (some custom kernels have a bug which allows the battery to completely drain, making it unable to charge again). Try buying another battery and check if it works.
After all this, if you still cannot flash the stock, your only option is Sony Service, and even they may not be able to do anything. Plus, if they get it working again, you may not be able to claim warranty if you're on a custom ROM, though you might get lucky, since your bootloader is still locked.
Hope this helps.