After my Observatory went gold it disappeared how do I get it back?

Sounds like a bad glitch. If it does not return when you restart the game, report it to customer support through the "help" button in the game settings (green gear icon).

Same happened to me but it was the Orangerie and I managed to get it back on my own. Try this: click the space where your Observatory used to be. The game should say the name of the object, as if it's there but invisible. Choose one of the older styles and close the style-picker window. Now you should see it again. Click your Observatory again and go back into the Style Picker window. Now choose the golden facade (if that's what you wanted) and close the Style-picker window. That's how I fixed it when my Orangerie disappeared.