Allegro PCB Design Tutorial
Creating an Auto backup Allegro .brd PCB File
It is possible that you loose the design content for some unforseen circumstances. Allegro allows you to create automatic backup of your design.
Go to setup -> User Preferences -> File Management -> Autosave

You need to check the autosave and specify a filename for the backup. By default the backup is created every 30 minutes. But you can specify your own time interval.
This youtube captures the complete sequence.