First of all, don't panic. Provided you've been saving your progress, you can recover it again. So the first step is to identify how you saved your progress before.
If you used a King profile, go to the settings menu and tap the orange King icon, then just submit the email address and password you saved when you set up your King profile. Once done, you should find your progress returns. If you'd like more guidance on how to create a King profile, a separate article about 'What is a King account and how do I create one?' can be found in this Help Center.
If you linked your game to Facebook, you'll just need to reconnect your game to the same Facebook profile using the blue Facebook 'connect' button. Important note: if you share your device, or have more than one Facebook profile, you'll need to make sure the active Facebook account on your device is the one you saved the progress to in the first place.
If you share your device and you're unsure of the Facebook profile you're logged in as, go to the settings menu and choose the option to log out of Facebook. Once this is done, check the active Facebook profile on your mobile and make sure it's the right one. Then go back to the game and connect to Facebook again.
If you didn't save your progress with Facebook or King, you won't be able to recover lost progress as it was never synchronized. Don't worry though, just create a King profile or connect to a Facebook profile and then get in touch to let us know. We'll do our best to get you back to the level you'd reached before you lost your progress!