I have been using my RPi3 B+ for a few weeks but one night when I was using it, it suddenly shutdown and the PWR LED was blinking so I disconnected it. Then I try to turn it on again and the PWR LED just turn on (without blinking this time) but the ACT LED didn't turn on. I made my research and I have tried replacing the SD card, I changed the charger and nothing happened. I also read that is a possibility that the board is dead .
I don't know if I'm doing something wrong or I'm missing something, but can someone tell me what I have to do to repair my RPi please.
P.D: Also can tell me why this happened.