I've also lost my prestige rankings and been reset to o level one , and my weapon attachments are locked even though the weapons are max level
For prestige ranks/levels: At the start of Season One, Military Rank and Prestige progression will be synchronized with Vanguard across titles. That means you’ll begin the season at the Military Rank or Prestige Level you’ve currently reached in Vanguard, or Military Rank 1 if you haven’t played Vanguard yet. From there, you’ll able to progress your Vanguard Military Ranks in Black Ops Cold War, Warzone, and Modern Warfare to unlock weapons, Equipment, Killstreaks, and other content for use in Vanguard and Warzone as you play. - Source: https://www.treyarch.com/game-intel/2021/12/December_6th_Patch_Notes
For core, non-seasonal weapons experiencing re-locked attachments: We're investigating this one as per the comments in this thread and this Trello card: https://trello.com/c/ivzkpzFD/264-weapon-progression-some-players-report-that-camos-and-attachments-are-now-locked