Sony just rolled out PS4 update 4.70 earlier today, and it seems like some people are encountering issues after updating their systems to it. Specifically, GamesRadar is reporting that a lot of players are encountering either locked up, or even bricked consoles, once they update to it. This… well, this can be an issue, obviously.
The good news is that for the time being, these players who ran into problems appear to have found fixes that have gotten them out of their problems. Basically, if you run into this problem, turn off the PS4 by holding down the power button. When you hear two beeps, the system has rebooted in Safe Mode. Reinstall the update, rebuild the database, make sure to have all external accessories disconnected (except for the controllers, of course), and make sure you are also not plugged into LAN. Restart the system, and you should be fine.
Alternately, if you have not yet installed the update, the best thing might be simply to not do so until Sony releases a hotfix for the problem.