Don't worry, it's easy to reinstall an app that has been erased. Just follow these instructions:
1. Open the App Store on your device. Make sure you are signed in with the same account used for the original purchase.
2. Tap on Purchased from the bottom navigation bar. (On iPhone or iPod touch, tap Updates from the bottom navigation bar, then tap Purchased.)
3. Locate the app in your Purchased list.
4. Tap the download button.