Progress Wars stores several cookies in your browser which it attempts to retrieve at each session to figure out your progress in the game so far. Among these cookies are the cookies that allow the game to retrieve your level and progress each time you open the game site.
If you cleared your browser's data recently, you will have gotten rid of the cookies. Also, your browser might be configured to delete cookies automatically when your session ends; check your preferences.
EDIT: Recovering your progress:
If your browser supports editing of cookies or if there exists an add-on which allows you to edit your browser's cookies, you can "recover" your progress, although it can be considered cheating.
- Install this add-on or something similar
- Tools > Cookie Editor
- Find the cookie listed for progresswars.com called level. Change the value to one below the level you were at when your progress was lost.
- Refresh and click the progress button. You'll be progressed to the next level and it'll be as though you reached the level through amazing dedication to progression.
- Tools > Preferences > Under The Hood > Content Settings > Cookies > Show Cookies and other site data
- Same process as with Firefox from here on in.