You must repair it, that the only way, don't ever flash in future if battery too low, anytime will hard brick your phone if battery dead in wrong timing.

Use the SUS and get it reflashed again. You must never let it die during a firmware update. It'll mess up the system.

Connect the phone to the computer and update it using the Sony Mobile Update Service.

Try to get it charged again till the led turns green, then try the SUS. You don't need to get into the phone system, it'll work when the SONY logo pops up.

just charge it and use Sony update service to fix your phone 

Can you charge it?

If ur phone still can turned on and pop up sony and restart always mean ur phone not yet hard brick.. Only soft brick.. So just connect to charger and left it there for 2 hours.. Within this 2 hours, yea, mac do have sus.. So prepare and install it first.. After 2 hours, make sure phone is turned off and follow the instruction in sus..

If u still can't understand how to do it, no one can help u.. guide is given wdy.. or recommend u go service centre and left them for few days if u would..

Sony bridge mac? http://www.sonymobile.com/gb/tools/bridge-for-mac/

If there is a green light then should ok le.. Another fast way is using flash tool and ask ppl upload u the Malaysia FTf.