Ooops! I think one of the documents I've been working with is lost. How can I retrieve it?
First line of defence is to check your Auto-Save setting in Mellel's Preferences. For most users, the option set by default is using the system standard (with Lion or better). The system standard version allows you to browse through past versions of a document, so you can recall a previous version of the document.
If that fails, you can go to the backup folder Mellel keeps. In the Preferences, under Open & Save, you can access your backup folder, and look through the files there to find your missing document.