I have lightroom 3.6, and yesterday, all but 16 of my 160 folders disappeared. How do I find them and reincorporate them?

When I attempted to turn on my computer, for some reason it wasn't responding so I turned it off and then back on.  My computer then was running fine, but when I brought up lightroom, I was shocked to find that all but 16 of my folders had vanished.  Any ideas what happened?--and more importantly, how do I find the files and reincorporate them into LR?

Thanks!