Sony has confirmed that it's working on the freezing issues that some users have having with the PS4's new Rest Mode.
We're aware of issues happening post PS4 2.00 firmware, including some freezes and boot errors. Working to resolve ASAP, will update soon.— PlayStation (@PlayStation) November 2, 2014
Unfortunately some users are reporting that when using Rest Mode the system won't turn on again, although it's easily fixed by holding down the power button until the system powers down and then turning it on again.
In the meantime it's best just to power the system down completely.
We expect new firmware will arrive sometime this week to fix the issue, possibly even as soon as tomorrow.