This can be caused by multiple components. It might be the case that you just have a malfunctioning display. In order to boot up the SIM card the phone needs to unlock it (by typing in the sim code). Also in order for a computer to recognise it the phone needs to give permissions, also by unlocking the phone. If the problem aint the LCD display, it is highly likely to be a problem with the logic board, which I wouldn’t recommend bringing it for to a repair store. The costs of repairing the logic board wil likely be more then the value of a new S6.