Look at the bottom of the list of clients in the home base view.... It may have been moved...

Try opening your computer's file explorer and see if the client shows up in the ProSeries' client data file.

There have been reports of this happening, I had it happen to one file, just poof, gone, no explanation.  I would definitely try the 2 ideas mentioned above first, and check your backup source, hopefully its got a copy saved for you as well.

If you accidentally deleted or changed the last name and saved the file, the client will show up somewhere else in home base. Try sorting home base by file name.