Why won't my PlayStation 4 turn on

I recently bought a ps4 off of the app let go. The guy was honest and said it shuts off at times so I said ok I can deal with that when I brought it home it would not turn on so many people told me it could be the power supply box so I replaced that. Last night I got the PlayStation to come on but it spit the game out of the system and would not let me place the game back in. Today I tried it and now the game will not come on at all but when I plug the power cord in the fan will start running but there is no sign of the ps4 coming on. I've done dusted it all the above can anyone please help