By default, OptiSurface Designer automatically saves a backup of your files every 10 minutes while you're actively working. If, for example, you're working on a file named MyDesign.osd, the automatic save creates a file named MyDesign.osdbac in the same folder.
To recover your work from the time the last auto-backup was performed, simply open the .osdbac file in OptiSurface Designer using File > Open.
If you've never saved your .osd file the auto-backup file is stored in the folder you last saved an osd file to or opened from. After saving your OptiSurface Designer file, the auto-backup file will be created in the same location as the .osd file.