When ever my chip bag gets left open are they just going stale, I set the oven to around 350, put as many chips as I want to eat on a baking sheet and stick them in the oven til they are warm, and viola, crispy chips :)
Pre-heat your oven to 375 degrees. Line a cookie sheet with foil and place tortilla chip in a single layer on the cookie sheet. Bake for 10 - 15 minutes or until desired crispness. Serve immediately.