For Apple and iOS users:
Go to the Apple App Store, and check if you can open the application through the App Store. If you can open the application from the App Store, it's possible that the application has been moved into a sub-folder on your device. Open the spotlight search by swiping-down anywhere on the home screen, then search "Todo." You should be able to locate the sub-folder containing the application. Once you find it, you can move it back to the home screen.
Android Instructions Coming Soon!