We got an error message on the TV last night

In a nutshell: There was some kind of error on the HDMI connection between your TV and your receiver.

HDMI outputs a perfect digital copy of the program you're watching, so the signal is encrypted. The TV and the receiver have to exchange a security "handshake" before you see anything. That's HDCP. Wikipedia can explain it better than I can.

If for some reason the two devices lose touch, you may get that error. You may not be able to tell whose fault it is - the TV may have a brief glitch, or the receiver, or it could be a bad HDMI cable - but what you did is the proper process to fix it. Power-cycle everything and the handshake is renewed.

If the error recurs, the steps to follow (in order of severity), would be to double-check your HDMI plugs, try a different HDMI port on the TV if there's one available, replace the HDMI cable, then swap the cable receiver.

Also, do you have the HDMI going direct from the box to the TV? Or does it go through something else, like a entertainment system? If so, sometimes an issue can happen there where ingress leaks into the HDMI repeater in the receiver. There are also different settings.