How do I restore DLC content in Oblivion Game of the Year edition?
Solution 1:
When launching Oblivion (or Skyrim/Fallout for that matter), the games usually load into a Launcher window. This window gives you some basic options for the games, like video/audio quality settings, support links, and most importantly here, Data Files.
Click into Data Files, and you will see a list of all .esm and .esp files that are being loaded into Oblivion. This is called your Load Order. Make sure that all necessary files are check-marked, including Frostcrag Spire. It would not surprise me if the game deselected it for some reason.