Sorry, but you will have to manually rebuild it. The good news is that you have the existing TOC to model after.
Helpful and Handy Links
If you have a backup of of the of your old build all you need to do is copy the *.hhc file into your new build. I also found that copying they files listed below pretty much rebuilds my whole project after importing the old pages. Also, recreate your folder structure before importing because they will not be created automatically.