1. I went to BlackBerry - BlackBerry | Wireless Handheld Devices, Software & Services from Research In Motion (RIM) and downloaded the Yahoo Messenger!
2. I then downloaded the Yahoo Messenger Service book at Box.net - Free Online File Storage, Internet File Sharing, Online Storage, Access Documents & Files Anywhere, Backup Data, Send Files and and then replaced the service book from my device/cell phone with this new Yahoo Messenger service book.
3. Now Y!Messenger works fine but my Yahoo Email icon disappeared. Before I was able to receive Yahoo Emails via my cell phone. Now I can't anymore.

Can someone help me?

Try going to Options >Advanced Options >Host Routing Table >Menu Key >Register Now.

If that doesn't do it use the "setup internet email" icon to resend the service books or go to your BIS email site on your PC and send service books from there.