You need to connect the box to your tv via Composite (Red, White, Yellow), in order to set the output of the box to 720p. If the screen goes black when choosing 1080i and stays that way, that means your hdtv is not compatible with that setting.
It does help to post the manufacturer & model of your set, or even if you have the book, open up to the specifications page in the back, and it tells you what resolutions that it is capable.
Most newer sets made after 2007, will do 1080p, even though they state that they are only 720p Native.
As long as you have the box connected via HDMI, the box should have seen that your tv was not 1080i capable, and set for default of 720p.
Now of course odd things have happened, and found out that the person having this issue, had changed the input on their tv to a different one, than what their Set Top Box or DVR is connected to.