My guess is a Window Focus issue. When the Print Where Dialog (PWD) comes up the selection criteria screen is still in the background and displays the last customer/vendor in the list. I suspect that the PWD is hidden behind another window, or off the edge of the screen or something.
 
Are you familiar with the old keyboard window controls*? They still work as late as Win8. I would try that to see if you can move the dialog around until you can see it. Hopefully once you've relocated it, it will be there next time.
 
If you can't locate it, I would suggest just pressing "Enter" after the last Customer/Vendor is there frozen for 10 seconds or so. That should activate the default print method.
 
*press Alt, then Space, the letter "M", and then the arrow keys will Move the box around. Be careful with this method - I would go a predetermined key strokes in one direction, and then back to where you started, before you try another direction, or the PWD could end up in Timbuktu. The other thing you could do is, instead of "M" for Move, type "X" for Maximize.

Everything worked fine on Friday, but now it's Monday and back to same problems.  I can run AP & AR Ageing and F2 will bring up customers.  
 
But when I go into AP-A & AR-A I cannot get the names to come up. Alt-Space-X is not working this time. The same for SO-A. I can't get SO or Customer to come up. I also can't get any s/o # or customer # on AR Payments.  The small box comes up in the left corner to minimize but nothing else is available.
 
I did a hard reboot and nothing changes.  We did not have any updates on the weekend.  I can't figure out what would have changed.  As usual, all other users are fine.