i was playin fallout 3 yesterday, playing through the Operation Anchorage X-pac. i had completed all the missions, gotten out of the simulation pod and went to collect my reward from the armory, then left the outcast base to screwed around for a couple of hours, then i turned off my X-box and when i turned it on today to play fallout when i went to "continue" in the main menu and it said, "this save file relies on content that doesn't exist." "Do you want to continue loading?" so i did and when i got on my character i had none of the things from the X-pac that i had collected.
You downloaded the content onto your Hardrive while it was plugged into another console.
you went home and played it some with live functioning.
when you turned on the xbox recently, live was not connected
if this is at all like what happened to you, then the only way to be able to access the content on you console offline is to sign on to your live account online and request to have your downloads re-initialized (you can only do this once per year without having to ask for special permission) Then, hook up both your Hard rive and your console to live, and re-download the DLC (should happen really fast)
Once done, the problem should be dealt with.
This is, of course, assuming your problem is anything like mine.