You can retrieve WhatsApp messages from iPhone backup:
To recover your chat history from an iCloud backup, first verify that there is an iCloud backup for your chat history in WhatsApp Settings > Chat Settings > Chat Backup. If you can see when the last backup was performed, simply delete and reinstall the app from the App Store. After verifying your phone number, follow the on-screen prompts to restore your chat history. The same iCloud backup requirements apply for iCloud restore. In addition, the phone number used for backing up and restoring must be the same. I.e., you cannot restore the chat history from another WhatsApp account. If you share an iCloud account, this will allow you to keep your backups separate.
Read more: http://www.whatsapp.com/faq/en/iphone/20888066