If you send us the user dictionary data file, there's about an 80% chance we can recover the data from it for you. To do this, connect your Pocket PC to your desktop, open up My Computer and Mobile Device, and there should be a "PlecoDict User DB C-E" file inside of there; drag that to your computer's desktop and then send it to us as an e-mail attachment. (if there's more than one "User DB" file, send us all of them)
In the future, it would probably be a good idea to make backups of this file regularly - to do this, simply repeat the procedure above but instead of copying the file(s) to your desktop copy them to some other safe location on your hard drive. That way, if you ever lose the file you can simply copy the uncorrupted version back to your Pocket PC to restore it.