Thanks Wayne but I have the latest version of OS and Numbers according to the App Store. It tells me that there are no updates.
For reference, I have a mini running ML that can handle these files just fine, although to be fair the versions might not be exactly the same (this machine is remote from me). In fact I used iTeleport to run numbers on the mini with the same data files last night so that I could keep working.
I think that it is a configuration issue of some sort on my local machine. When I upgraded the mini to ML, I did a clean install while on the pro, I updated in place. Maybe I need a clean install but I was hoping to avoid that.
I had already deleted all preferences and reloaded numbers, but I did try your suggestion. I created a new user and opened numbers. When I created a new file I had the same problem with the white pane and the data there but hidden from me. Again I could click on cells and see values and formats but the pane stayed stubbonly white. Thanks for your help but I don't think this is going to be easy unless there is a security setting or something that hides the main pane until you issue some magic incantation.
This is all very strange but my Numbers issue is now resolved.
Although I had tried to solve my problem by making changes and restarting (warm reboot) multiple times, nothing had helped.
However, I just closed all my apps and then did a complete shutdown. Next, I powered up using the button on the front of the Pro (cold reboot) and all is well.
I have no idea what was going on but I was having other problems that I described elsewhere in these support discussions and they are now resolved also.
For reference, those issues were:
choppy transitions between desktops and especially swiping to the dashboard,
choppy animation when opening the Apps folder from the dock,
opening notification center was very choppy and showed red streaks,
AutoCAD LT wouldn't load indicating that my system was unsupported (not true when all is well again),
So, it seems to me that the graphics subsystem was not being recognised correctly before the reboot and that was causing all of my problems. The screensaver had also stopped working being replaced by a screensaver consisting only of the text, "Quartz Composer." I expect that will also work now.