Hi!
Well, could be multiple things!
First things first, are you able to boot the console in Safe mode? If yes, please restore the PS4 from here. Update it or completely reinstall the software.
If you can't boot into safe mode, then it could be the APU or the power supply.
Replacing the power supply is the first thing to do (after you tried booting into safe mode and tried a known working harddrive, if you have one).
Guide: [guide|89196]
Please let us know how it goes! Good luck!!