If your phone is turned off or if you’re outside the wireless coverage area and someone sends you a text message, it will be delivered to you when your phone is switched back on or when you’re back in the coverage area.
Messages are stored for delivery for up to 3 days (72 hours) so it’s best to turn your phone back on or ensure you’ve got coverage within this time for proper message retrieval. After 3 days, your messages cannot be delivered to your phone anymore.