Text messages disappeared on my iPhone 12

You may do some of the suggestions below.

1. Check your iCloud settings: If you have enabled iCloud backup for your device, you may be able to restore your messages from there. Go to Settings > iCloud > Manage Storage > Backups and check if your iPhone 12 has been backed up recently. If yes, you can try restoring from the backup.
2. Check your Message settings: Make sure your Message settings are set up correctly. Go to Settings > Messages and check if the "Message History" setting is set to "Forever." If it is set to "30 days" or "1 year," your messages may have been deleted automatically.
3. Check if your messages are on another device: If you have other Apple devices, such as an iPad or a Mac, check if your messages are synced to them. Open the Messages app on the other device and see if the messages are there.