Point to note on the above reply .
We on XDA first found that users left phone on charge overnight where reporting a dead unit . After a number of these posts it was recognized as what became SDS or sudden death syndrome .
Leaving on charge overnight is not the actual cause of SDS but something i don't risk any more fixes or not .
SDS seems to manifest in 2 distinct ways.
1) Complete eMMC failure. This usually happens over night whilst on charge* - The LED may be lit but the phone will not wake. Subsequent removal of the battery will turn off the LED. You will never get the LED back on. The phone will never turn on again. It needs to go to a service centre and have the motherboard replaced
* Being on Charge is NOT the cause. This is a writing to memory issue and it is more likely to happen when teh device is awake rather than in deep sleep. When the S3 is on charge, it holds a wake lock (so remains awake) throughout the entire charging period, meaning SDS is more likely to occur when on charge This does not mean you should avoid charging your phone. If it's going to happen, it's going to happen