You generally cannot see messages that were unsent or deleted on Messenger
Check your notification history if message previews were enabled
Check email notifications if Messenger alerts were sent to your email
Review any chat backups or device backups you may have
Ask the other person to resend the message
Check if the message is still visible on another device where you were logged in
Use Messenger’s Download Your Information tool to export your data
Review archived chats in case the conversation was not deleted, only hidden
Check linked Instagram messages if the conversation was cross-platform
Avoid third-party apps or websites that claim to recover deleted Messenger messages