Answer #1: Dust automatically deletes all messages after 24 hours.
Answer #2: Have you deleted your app? Dust is end-to-end encrypted, so if you delete the app from your phone, your encryption keys are deleted as well. Once you reinstall the app your Chat rooms will reappear, but your past messages will not. This is a security feature and unfortunately is not reversible.
Answer #3: Have you signed into a new device? If so, due to Dust’s end-to-end encryption you will not be able to receive your previous messages on the new device. This is a security feature and unfortunately is not reversible.