Firefox should not have reset itself. Was anything going on at the time? Were you having any issues? Firefox does auto save things for you.

Open a new window or tab and type about:support in the address box. Under Application Basics look for Profile Folder and press the Show Folder button. Look for a bookmarkbackups folder.

Could you check your desktop for an "Old Firefox Data" folder?

If you are very low on disk space during a reset, the old folder sometimes remains in its default location instead of being moved to the desktop. You can check by opening the Start menu and typing or pasting the following in the search box and then pressing Enter:

%APPDATA%\Mozilla\Firefox\Profiles

Can you find a second profile folder here in addition to your currently active on? (Usually the time stamps on the files tell the tale)