A: Did you actually save the city? There's no autosave in SimCity 4.

B: Are you sure you didn't save your city in the tutorial reigon?

Assuming you saved the city, you could always go to the regions folder and try to use Windows' recovery options to retrieve prior versions (under the folder's properties menu, furthest tab to the right).

Otherwise, it's probably not retrievable. The best advice here would be to avoid spending too much time in Timbuktu because it's been know to randomly reset, and isn't the most stable region. A custom region or any of the game's other prepackaged regions would be a better choice as they aren't prone to random resets for the sake of the tutorials.

In fact, there's a blank region shaped just like it, isn't there?

I don't think so...You could just copy the region and make a new one based on it that wouldn't be overwritten every time the game decides it wants to reset the tutorials.