Jericho Jared Apr 24, 2012
Console Commands... that's it.

If you had sold it to someone, usually it will stay in the shopkeeper's inventory indefinitely (after all, who's going to want to buy a mask that tells them to take a bath in battery acid when they put it on?). If you just ditch it somewhere that you have no affiliation with however, it'll be gone for good.

If you're using the PC (unlike me), you can spawn a new mask into your inventory, but the only method for the consoles is to load a save where you had it, or start a new game. Sorry.