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have you tried connecting it to a pC? are you using the wallmount charger?
if the device is still not responding try to have it checked at your local service center Local support
it could be that the phone it's off, or that they have it on airplane mode, if they have it on w/ a SIM card, it could have a great network coverage and NO other apps running, there are myriad reasons/variables.
Airplane mode will speed the charging process, if the phone it's off it will be even faster.