I have a Motorola DCT 6416 III. Periodically I go through the following cycle with it.
1. I go to On Demand channel.
2. Hit Select Button, to get to menu. Nothing happens.
3. Hit it a few more times. Still no menu. Sometimes the words "Network Unavailable" appear, sometimes not. Sometimes the remote light on the DVR that shows you are pressing a button on the remote will remain constantly on, sometimes not.
4. At this point, the unit will no longer do anything... I can't change channels, etc. I have to power cycle.

I recently swapped boxes, but the problem persists. I have Cox Cable.

I've got a better handle on what is going on now, so I'll post it just in case someone else is similarly perplexed someday and finds this thread. The problem I am having has nothing to do with my box; it comes from my cable company's buggy On Demand server. When you try to go into the On Demand menu and the server is overloaded or not working right, you get a couple of minutes period where your box is trying to make contact with the server and won't accept more commands. If you wait it out it will unfreeze... I was power-cycling it earlier than I need to.