my lumia 920 wont start even though the charger is plugged in, why?

my phone had been working really slow since the last few hours so i turned it off planning to restart it, the battery at the time of shutdown was 7%, once turned off i pressed the power button again, the lights lit up, and i went to the kitchen,i come back its black again, so considering the battery drained i plug in the charger, and its still black, not even the charging sign on the empty battery is appearing. help please. is it bad?

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