IMPORTANT: This is a first draft of this page. The information below might be useful in some circumstances, or it might not be. I'm really hoping other people will add to this page!
First suggestion: have a backup program which backs up your data at regular times to an SD card. This is by far the easiest way to restore from a crash. Suggested programs: Backupman, Resco Backup, BackupBuddy VFS - they all provide extensive trial periods so try them before you decide which one to buy.
One Datebk user reported that notes attached to appointments had disappeared. This might have been caused by a problem with the hotsync conduit, or the notes might have been deleted by accident or by another program on the Palm or on the PC. If something similar has happened to you:
Check to see if the notes still exist on your PC. If so, hotsync with the appointments conduit set to "desktop overwrites handheld". Be aware that this will remove all changes you might have made on your Palm since the last hotsync (if that's a problem, just copy the notes from the appointments to a text file on your PC, sync as normal, and then paste the notes back in on your PC and sync again).
If the notes are blank on your desktop too, and if you have made backup(s) of your user folder on your PC, then check the datebook database in one or more of the backups - you can probably open the DatebookDB.pdb file in a text editor and search for the descriptions of the appointments. Note that you will see a lot of binary data (strings of unusual characters and seemingly random letters and numbers), but you will also see the text of your appointments and their notes. Using a text editor like this means that you don't have to import the backed-up database into your Desktop or Palm, which means there are no concerns about overwriting your current data.