If you were able to "save as", then you were definitely playing in single player mode. Make sure that when you restarted, you didn't just accidentally hit the wrong tab and are viewing the multiplayer mode.
The most likely reasons for not finding your saved games is that you are not logged in as the same user, either in Windows or in SimCity (or you're on a different computer). The single player mode saves your games to the hard drive of the computer on which you're playing.
First, make sure you're using the same user name in SimCity. It's easy to accidentally log in to Origin & SimCity under another email address, and that makes it look like everything is gone. If that's the problem, it's pretty easily solved and panic averted.
Next, make sure you're logged into Windows as the same user; the saved files are under the Documents folder for the user name you were using. If you want to switch the games to a different Windows user name (or computer) - First, log out of SimCity and Origin. Go to the original location \Users\<user name>\Documents, copy the entire SimCity folder, then paste everything to the correct user name's Documents folder.
When you launch SimCity, it will find the folders and show you your cities. There should be a "resume" button with the city you were last playing, and a "play" button that will take you to the list of your "save as" games and another tab if you want to start a new region.
Hope that gets you back to building! Good luck!
