Whenever I open a drawing it used to have a window pop-up that asked for the drawing reference or relation drawing, the box no longer pops up and the drawing now just has the boxes with lines through them. How do I get the window to pop-up again so I can access those drawings?
Go to Tools > Options > System Options > Messages/Errors/Warnings and look for message under Dismissed messages. Select the message and and click OK. Next time you should see the message for the lost reference.
You might also find this post helpful for your future reference.