My game seems to have restarted itself - I lost all progress. How do I get back to my old game?

When you type into the URL bar of your browser, you are automatically redirected to the server that the website thinks you are located in, based upon your location. When you go to the login page, at the top of the screen you can see a small flag. This indicates the server that you are visiting. Some languages have more than one server (English, Spanish and Portuguese), so make sure that you're in the right place or just pick the correct flag from the list.
Another reason might be your inactivity. if you haven't been playing for the last 30 days, your accounts might have been removed from the game world.