Missing sheets

I work for a law firm and we have been using/editing a particular Excel workbook for over five years. I has/had numerous worksheets. After we went through some server/backup changes (may or may not be pertinent) we noticed that some of the older sheets of the workbook (those mainly used/edited in recent years) are missing. All of the ones we currently use are still there.

I've read about the hidden sheet issues of some but that appears to be an easy fix and usually is all of the sheets. This is about 30 out of 50 sheets that just vanished.

Any thoughts on this?

I have Office 2013 that has been updated to version 15.0.4815.1002.

Thanks for any help/insight.

Success! Found the hidden sheets and marked them to be visible. I appreciate your time and help.