I made some column bar graphs and all was fine. Then, suddenly, the bars vanished from my graph. The axes were still there, but no bars. The data tables are fine, the Y axes had sensible limits, but no bars. What happened?

We've seen this once with Prism 3.02 and can't explain why. But we do have a workaround. Go to a data table, click analyze and choose Column Statistics. Now your bars should reappear. No, we don't know why.