After tapping on a coin, you need to move your device around to find Garfield -- instead of the screen turning black, it is actually activating your device's camera, so you can look for Garfield using Augmented Reality.
You should see whatever your back camera is pointing at, and an arrow pointing to where Garfield is (unless you already see Garfield), so you need to turn in that direction while holding your device.
If you only see black, there are 3 common possibilities:
1. Your rear camera may be covered -- please make sure that nothing is covering your device's rear camera.
2. You might have turned off the required permissions for Garfield GO in your Settings app -- please allow Garfield GO to access your device's camera.
3. Your device might not be compatible with AR Mode -- If you are past the tutorial, disabling AR Mode might help. Go to [Main Menu] --> [Settings] --> Uncheck [AR Mode].
If after trying the suggestions for all 3 steps listed above, you are still not able to see anything other than a black screen, please submit a support request (there should be a "Submit a request" link at the top-right corner of the page) with your device's model and currently installed Operating System version number.