MACBOOK PRO STUCK IN TARGET DISK MODE

Hello:

I wanted to connect my MacBook Pro (retina 17”; late 2016) to my MacPro using the MacBook Pro display for MacPro. However, the Target Disk mode did not launch completely, and has now a dialog box on my MacBook Pro showing a lock, and window to log in a password. I did enter the password for the MacBook Pro several times, and then one for the MacPro. The display is frozen on a dark grey background with the lock and no other actionable items on it. How do I get out of this mess. I did try to start in safe mode, tried starting for an external drive, and using an external drive as a start up disk. nothing works. Could it be the password I am being asked to log in is one for the firmware? How do I resolve this? Your help is greatly appreciated. Thanks, Oscar