When using any version of AutoCAD with two monitors or a monitor and a projector, it's possible to position any dialog box on the secondary monitor.  This works great until you have to go back to one monitor for whatever reason.  (It does happen, particularly after using a projector).  If you find yourself with missing dialog boxes, you can reposition them by editing their position in the Windows registry.

Workarounds:
Note that all of the standard windows registry warnings apply:  You can severaly damage your system if you edit the wrong system setting in the regitry.  Only people very comfortable with registry editing should attempt this workaround.

• Shut down AutoCAD
• Start the registry editor and navigate to hkcu\software\autodesk\autocad\rxx.x\acad-xxxx:xxx\profiles\youruserprofilename\dialogs
• Under the dialog tree, you'll see a list of all of the AutoCAD dialog boxes.  Find the one that has disappeared from your screen and directly edit its location and/or size.