If you have backed up your iPhone whatsapp messages to your iTunes or iCloud, then you can easily restore whatsapp messages from iTunes or iCloud.

But if you don't, I don't think you can recover deleted whatsapp messages from iPhone unless you try some professional iPhone data recovery like Mobiledic iOS Data Recovery!

If you are using a Mac then the password may be stored in your keychain. If you are using windows then unless you can remember it its gone. Other way is try iSeePassword iTunes password recovery program to unlock your iPhone backup.