I can tell you what worked for me, but it may not work for anyone else.  Go into another directory (for instance we have one named payroll) and open a client in that different directory.  Then go back to your main data directory and see if you can open clients in that directory now.  I hope that it works for you.  I have 7 computers and only 2 of them work properly with the software.  All computers are Dell and all are on updated current Windows.  There seems to be no rhyme or reason.  Good luck to all.