Make sure the correct company file is open - often when that much data "goes missing" the issue is someone accidentally opened a very old copy of their company file. In the Mac OS, use Spotlight to search for QuickBooks files. If this is the Mac version of Quickbooks, you can search for .qb and find all the files.
After that carefully look at each file. You can select the file in the Finder and choose File->Get Info. Look at the file size. The largest file is likely to be the one with the most data in it.
Within QuickBooks you can see which file is open by choosing Command-1 (Or Help->Get Information). At the bottom of the window you'll see the file that is open and the path to it in the Finder.
Finally, if the biggest file is not the one that is open, try opening the biggest one using File->Open and navigating it to it.
The other possible issue is data damage, although that is much more rare. To see if that is the case try File->Utilities->Rebuild.
