Hi Itsjessy06,
We understand you're having issues logging in to your Facebook account through the app. Have you tried another Wi-Fi network, or while connected to your cellular network? Are you able to log in on the website using Safari? If so, that lets us know this issue is isolated to the application. If not, the issue would be with your account information and you'd need to contact Facebook for assistance.
You may need to update your software if it's outdated. Be sure to create a backup before updating your iPhone. These articles will help:
How to back up your iPhone, iPad, or iPod touch
Update your iPhone, iPad, or iPod touch
If this is only happening with the Facebook app and you're able to use all other apps successfully, follow the steps in this article to report this behavior to the app developer: How to contact an app developer.
Hope that helps!