Both expressions are correct in terms of grammar. Which one you'll use will depend on what do you want to say. If you say "I was lost" that means this happened in the past. It's something that happened and ended. For example: I was lost in the city this morning. If you say "I will be lost" this means you are predicting that this will happen in the future, but it hasn't happened yet. For example: I don't have a map, I will be lost in the city.

Both are correct but they are different tenses. "I was lost" is past tense, and "I will get lost" is future tense.

Thank you.