What is wrong with my phone (please help)?

My phone has been messing up for the past few weeks now (replacing letters, resettin itself). But today, my battery was low so I turned it off. When I went to go plug it into the charger, the phone turned on and says it's normal "SGH-T139" but then, five seconds later "3D10017" appears above the letters in red and the keypad flashes constantly. I can't turn it off without taking out the battery, and even that doesn't help. What's wrong with it? Will I have to take it into T-Mobile? And will they even look at it since I bought it from Walmart(verizon never would)? Please, this is urgent, I need this phone, it's my only means of contact and all my numbers are only saved to the phone, not the sim card.
 
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