Not 100% sure that this is what happened to you, but this has happened to me in the past:
I'm doing something with my sim or in buy/build mode and all of a sudden my game freezes. The sound is still playing but time is stopped and there is no response with my sims. I can't click anything. When I alt-tab to minimize the game, I check my task manager and look at my CPU Usage. It's 100%. It also says that The Sims 3 World Adventures (since I click the WA shortcut to play) is Not Responding. Now, this usually happened when I was changing between speed 3 and 1 on my game, but not always. It also didn't always happen on a lot with a whole lot. It happened to me when I was playing the Perfectly Happy Challenge with my single sim on the lot with a very small house.
I'm thinking what might have happened to you was because you were in buy mode changing to live mode, and you had dealt with the garden previously, if you had anything else on your computer running (even applications that automatically start when you boot up your computer and don't show up in the bar on the bottom of the screen) your computer had too much to handle and was basically overloaded. When you shut down sims, it was fine and when you started sims up again, it was till fine because you weren't making it do what it did before. This could have been a freak accident thing and may never happen again, but you never know.
