I am not entirely sure if this will help, but try searching your computer for your Sims 3 folder?
Click the Start button > and type in Sims 3 to see where the search takes you to see if you can find any of your files that way?
Question, how are you backing the Sims 3 folder up? Do you for example, cut/paste folder to desktop or do you copy/paste folder to desktop.
What I've been doing lately is copy/paste folder to desktop for temporary and then copy/paste it to my usb key. Check that it's there, and check again lol to be fully sure all is there.