Before putting the box into standby try to ensure that you change to a Freeview channel rather than an ip channel.
See if that makes any difference.
Also are you using a UHD TV ?
Well first thing to check is that the Hdmi output setting on your Youview box is compatible with the TV you are using. The default that the UHD box is set to is a UHD setting which would not be compatible with the TV and would not display appearing as a black screen.
As for using the box with no aerial. Using it without an aerial is generally ok BUT the initial setup process involves some stages that involve checking eligibility for using some of the Youview players based upon where you receive your Freeview transmissions from.
This happened to me as well. Have only had box for a few days. 1 failed recording and had to hold power button. Not sure if it was left on an ip channel... Have set time out to 12 hours.