Hey Karen5591,
We'd be happy to help out. Sounds like you believe your Messenger app got deleted, but perhaps it was just removed from your Home Screen. If you're not seeing Messenger on your Home Screen, you can search Spotlight to see if it's still in your App Library by just swiping down on your screen anywhere but the very top, then type in Messenger. If it produces a result, this indicates the app is still installed, it's just in the App Library instead. You can hold down on the app in your search results to bring up the option to "Add to Home Screen.
If you search for the app and it produces no results, this indicates the app is no longer installed and you can redownload it from your purchase history using these steps:
How to redownload apps on iPhone, iPad, or iPod touch
1. Open the App Store app.
2. Tap the sign-in button or your photo at the top of the screen.
3. Tap Purchased.
4. If you use Family Sharing, tap My Purchases or choose a family member’s name to see content that they purchased.
5. Tap "Not on this [device]."
6. Find the app that you want to download, then tap the download button .
Learn what to do if you can't redownload the app. If you can't find the app on your device, use the App Library to find the app.
All this and more is available here: Redownload apps and games from Apple - Apple Support
We hope this helps out.
Take care!