Don’t worry, losing one word isn’t the end of the world! Since the BIP39 word list contains only 2048 possible words, you can systematically check for the missing one. There are tools that help with this, and one of the best ways is using this BIP39 word list, which lets you verify and reconstruct your phrase. You can enter your 11 known words and check for possible valid combinations. However, be careful and always use a secure environment when dealing with private keys!