A common troubleshooting step for mobile games is uninstalling and redownloading the app. Your progress should be automatically restored after you reinstall PvZ 2, thanks to the iCloud backup. You almost certainly have iCloud enabled on your device, but you can check to make sure:
Important: We strongly advise against signing into other people’s iCloud accounts on your device.
PvZ 2 syncs your profile and game state to your iCloud account. If someone logs you out of your iCloud account on your device, then logs in with their iCloud account, the game will sync from your device to their iCloud account, granting that person permanent access to your profile information. This could result in their deleting your entire game.
You can log into different GameCenter and Facebook accounts on your device with no risk to your data integrity or security. It is only the iCloud login within the device's Settings menu that will cause these issues.
A deletion on one of your devices is a deletion on all of them.
Let's say you have an iPad and an iPhone, and you're signed into your iCloud account on both devices. The iPhone is in your hands, and the iPad is in a family member's hands. They launch PvZ 2, and they accidentally delete your profile in the main menu. The next time you launch PvZ 2 on your iPhone, your profile will be missing.