Screen goes to black after boot

I have an Inspiron 8600 which is about six years old.  It has all the Microsoft updates installed, and has been scanned and pronounced clean by AVG and Spybot Search and Destroy.  The computer is mounted on a docking station, which was purchased from Dell at the same time as the laptop.

When I power up the computer, it proceeds past the POST and the welcome screen, and the Desktop loads normally, showing the Start button, the Taskbar and System Tray, and all the icons on the wallpaper.  At that point, the screen turns completely black, and using the mouse or keyboard does nothing to restore it.  However, if I connect a remote monitor to the dock, the video is displayed normally, and I can use the laptop just fine.  When I use Control Panel to check the Power Options utility, it shows everything set to "Never," and that Hibernate has been disabled.

This problem started about one month ago, and I've been searching for a solution ever since.  Other than a few posts on the Internet suggesting that the battery may be bad (it isn't), I haven't read of any fixes.

Can anyone here think of a solution for this problem?