lost progress in candy crush saga. how do i get it back

This is probably better asked to Candy Crush support. Do they have an account for their game? Largely it’s up to the game developers to provide syncing and backup of content.

If they have nothing and you have a recent backup of your device you might be able to get up to that backup (meaning if you made a backup a month ago and you restore from that backup your device will be set back to as it was when that backup was made, so you may get your candy crush back but may lose any progress in any other game made since that backup was made)