xperia sp wont turn on topic

hello my friend, recently i got my phone out of my pocket. i realized it was off. i tried to turn it on but nothing, then i pressed the off button on the back and it went on. stayed on for a couple of minutes, then the it frozen, the screen faded and it went off again. i left it the whole following morning charging, when i came back it turned on, stayed on for a couple of minutes (a little bit more of the previous time) then again same thing. if i charges it has a red light (sign that it is charging) if i disconnect the charger and try to switch it on it flashes 3 red light then it stops, and then again three red lights. i tried to charge it via computer but it wont be recognized, i also tried volume up and power button, volume down and power button, and the off button on the back. at the service center they say it cannot be fixed because it is out of warranty since the screen as been replaced by unofficial service. after a bit of charging it turned on today, but soon went off. i need help, i would at least want to know what the issue is. thanks a lot. ps. i can manage to let it work