Suddenly my WD external 500GB My Book dropped off

Here are a few things to try. If you have access to a secondary PC or laptop, try connecting your drive to it using a different USB cable. Also try unplugging your drive from the power outlet for a few seconds and then connecting it again. If it's still not being recognized, go to the Seagate site (they bought Maxtor) http://www.seagate.com/ww/v/index.jsp?locale=en-US&name=Operating_System_Detection_Issue&vgnextoid=1b734a3cdde5c010VgnVCM100000dd04090aRCRD

This will help with troubleshooting.

If that doesn't help, I would assume the worst and begin working on data recovery to be safe.
Try a free download of Seagate File Recovery.

You can preview your disc and file content in a simple hexadecimal viewer which tells you if a successful recovery is possible. You'll know in advance exactly which files can be recovered before you make a purchase.

If your files are recoverable, simply purchase the software for Windows or Mac and restore your files. It's that easy.

https://services.seagate.com/diysoftware.aspx