Accidentally deleted all my messages and I need them back

If you have iCloud Messages turned on they are gone. If you don't you will have to restore your iPhone from your backup. You will lose anything you added since that backup. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If you don't have a backup they are gone.

Messages are saved on your latest iPhone backup(unless you house them in iCloud). Restore that backup to retrieve the messages that were there when the backup was created.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support