I had that problem with the original Roku, which was due to HDCP handshaking. This was on a brand new Sony tv. Ended up going with Component, until I got one of the newer boxes, and then the problem went away.
It has nothing to do with powering off all of the networking equipment. Now on the other hand, if you have a catv set top, it can cause it to go into a secure mode, which means that you would have to call the catv to push a signal through to unlock it again, so that you can use it.