Share your experience!
i have bricked xperia l. Phone doesn't turn on and the fastboot and flashmode doesn't work, light doesn't shine anyway and computer doesn't see any connected device. Led is shining only when i connect phone with pushed power button or vol+ on some colour between yellow-green on a half second, it's strange a bit because i'm plugging device without battery.
did you recently modifies the software of the device?
i suggest you contact your local service center for repairs Local support
or post this question at XDA forums