Where is my dialog?
If you run the OPEN or SAVEAS command and you don't get the expected dialog box, but instead receive an invitation to provide the path on the command line... don't worry, you just need to change a system variable. Type in FILEDIA and set it to 1 to restore the dialogs.
FILEDIA controls whether or not FILE DIAlogs appear for commands such as OPEN, SAVEAS, EXPORT, RECOVER, and NEW.
So if you didn't set FILEDIA to 0, how did it get changed? Sometimes external (lisp, vba, etc) commands may set FILEDIA to avoid dialogs and fail to set it back to 1. Civil 3D itself is known for mysteriously setting FILEDIA to 0 also. The PUBLISH command also sets FILEDIA to 0, but I have only heard of rare cases where it doesn't get restored.
Along these same lines, there is a less used system variable for CoMmanD DIAlogs named CMDDIA. According to AutoCAD documentation, CMDDIA only controls whether or not the in-place text editor appears for the LEADER and QLEADER commands, but it also controls dialog vs. command line prompts for many AutoCAD Map commands. A setting on 1 enables dialogs, and a setting of 0 means you will be prompted at the command line.