Well I just recently bought a brand new copy of Oblivion for the Xbox 360, the standard edition since I'd had installed Shivering Isles 3 years ago....I put the disc in, and when it went to the main menu, I loaded my last save file which had 184 hours or so on it, Save 1157...it said something like 'some content is missing, some will not load when loading save file' or something along those lines, 'continue loading save without content anyway?' sorry If i'm not remembering the exact details...

so I selected load, and just to test things out I looked for the Knights of the Nine building and the Shivering Isles teleport and could not find it anywhere. I also could not find some of the gear I had gotten from Shivering Isles quests in my inventory nor chests...I know it's been well over 3 years since I last played it, but I specifically remember saving my last file in my own home in Skingrad...I loaded the save file, I'm in my Skingrad home, but none of my Shivering isles or Knights of the Nine armor is to be found...what the heck?

How could anything POSSIBLY be missing if Shivering Isles is still INSTALLED on my HDD? Do I need to install Shivering Isles again because I may somehow have an older version of Oblivion? If I do all that, is my save files corrupted since I loaded without any SI or KoTN content? Can I get my stuff back? I actually came back to finish about 10 or so more quests (100% completion on the UESP wiki actually says something like 199 quests in total in the game, and I've completed 189 quests) I like to complete any Elder Scrolls or Fallout game before moving on to the next one, or else it's impossible to play without thinking of the previous title.

The whole time I cannot immerse myself in Skyrim because I feel like getting the last couple achievements by completing the Dark Brotherhood questline, and finding out what happens in that questline, it's the only major quest line I had not completed and so on and so forth.

You may call me crazy for wanting to finish Oblivion before fully getting into Skyrim but just so you know I've put 115 hours in Skyrim already...haven't done any major quest lines in Skyrim however.

TL;DR; can I get my stuff back by installing the expansions again, even though I'm pretty sure I already have them installed? Why would it be missing again?

The expansions need to be activated in order for their content to appear. Just because they are installed, doesn't mean they're active. Activate SI and KotN, then reload your last save ( 1157 according to your post), and the missing content should be there.

how do I activate it?

hellooo?

I don't know how it is for Xbox, but I know that on PC, you have to install the expansion content after the main game, because it makes patches to the original game.

If you're on xbox, it would be worth going into your downloaded content or your saved things and seeing if Shivering Isles is in there. That would tell you pretty quickly what is missing.

Check you got latest patched installed. It helped me.

so putting in a standard copy of the game, brand new never un-opened, possibly an older patched version, means it overwrites the patch that the Shivering Isles applies, so I need to get Shivering Isles again?

It works that way on PC. I don't know how the patch is applied on console.

This exact same thing happened to me in FO3. I stopped playing about a year ago, then felt like getting back to the wasteland. Loaded my guy, content doesn't exist, yet is clearly there sitting on the harddrive.
If you haven't started either dlc, delete both expansions, then re-install. You should see them again, in game that is, with messages, like the Strange Door, or rumors about the prophet.

I doubt that this is your problem.

The difference is that on PC, you need to install Oblivion game disc as well as the 2nd disc.

On Xbox, you just put the disc in and then play.

I think that your problem would be accidental deletion of the SI and KotN. Go through your Xbox memory and check for them.

I would not make a thread like this if I didn't make sure the expansions were still there.

have you cleared your cache memory by any chance? (i have 360 version).it might be that the update has been deleted,the game doesn't recognise the extra content.try re-installing the games again (obviously,keep your saves).i found that the same thing happened when i cleared my cache memory.it just didn't recognise the extra content and apologies if you have already done those things (have just woken up,so not with it at the moment/nothing new there though

At the main menu, does the game say 'loading extra content'?

Knights of the Nine and Shivering Isles are supposed to be installed after you install Oblivion. I think that if you re-install Oblivion, you have to reinstall any add-ons for them to work. I guess it wouldn't hurt to try to use your previously-installed add-ons, but I'm not optimistic about it.

On a PC, the first menu you see upon launching Oblivion contains a Data Files option. If you select Data Files, you get a checklist of add-ons. Check the ones you want loaded, and uncheck the ones you don't. I do not know if Knights of the Nine and Shivering Isles would appear in the list if you have not reinstalled them after uninstalling and reinstalling Oblivion.