When Bluestacks was uninstalled, it asked if you want to preserve the data or not. If you choose Yes, then the next time you install Bluestacks, it should use that data with applications, games, and extra content (WhatsApp, images, contacts, etc). Unfortunately, it's impossible to restore the data if you don't have those files. Bluestacks should detect it automatically. Install Bluestacks, then install WhatsApp and activate it. During registration, it should ask you to use the old messages, contacts, data, etc.