Have you ever lost your AutoCAD open dialog box? I had this question several times. I never experience this, but it looks like this is a common issue. I found many similar thread in some AutoCAD forum. This thread is an example.
When AutoCAD ask us to open a file, either DWG file or other file type, we will see dialog box similar to this.
Enter name of drawing to open <.>: plan.dwg
Some of them said that the dialog box is missing after their computer hang or freeze. I suspect they were running some LISP or program when it happened. Or cancel poorly written LISP without error trapping.
To restore it is easy. Somehow, an AutoCAD system variable has changed.