Hold down the info button on the remote until a menu page comes up, then immediatley hit 1379. This will reset the DTA but you will more then likely need a "HIT" sent by calling comcast. In the prompts before talking to a rep there is an option to send hits to your equipment. So you can try this before speaking to a rep.
I have see that when first activating the channel stays "frozen" on 5(here in colorado) for 30 secs or so before i am able to change channels normally.
If it still is stuck on 8 for you then try to reset and rehit the dta
I just received 2 DTA's in the mail in a self-install kit for my kids TV's and hooked them up last night. They sent the signal to activate them and after just a minute the TV's turned on. However, they both turned on to the same channel (QVC). They wouldn't change off of that channel to any other channel. I know the remotes work, when I manually enter numbers it shows up on the screen that I'm entering the numbers, but the display then just turns back to 8 - QVC on the bottom corner although when I push the up and down channel buttons nothing happens. I told the Comcast representative this and she stated that it takes up to 45 minutes for everything to download and to give it time so I left it and went to bed. This morning both TV's are still on channel 8 and they will not change no matter how many numbers I type in, this shopping channel is the only channel my kids can watch now.....any ideas?