I play the PS5 version in offline mode. This happened to me twice and I think I figured out the cause. Both times I played a random priority assignment that had missions in systems that were *not* unlocked. The next time I started the game all items except the stash were gone. Skills, perks and attributes were all reset and all the systems were locked.

I usually don't play any other activity until after the campaign. Since offline mode is new you have to do all the unlocks again, so that was my focus. I was planning on deleting the character after unlocking everything ... just had to delete it earlier than planned.