If you've accidentally deleted notes by bumping the delete button, you'll find the notes listed in a file called deleted.txt in your profile folder.
For bigger losses, Anki provides an automatic backup system, which takes backups of your cards and progress, but not your sounds or images. Please see the following pages depending on the Anki software you're using:
- Computer version: http://ankisrs.net/docs/manual.html#backups
- Apple devices: http://ankisrs.net/docs/am-manual.html#automatic-backups
- Android devices: https://code.google.com/p/ankidroid/wiki/FAQ#Lost_my_decks