External Monitor highjacking

Running Windows Home Premium on a HP 6400 laptop. An external monitor Dell 19 ") was attached and worked beautifully for a while. After a normal shutdown of the laptop the add-on monitor display was really off, horizontal bars across the screen and the display appeared to be somewhat dimmed. I tried to switch back to the laptop display using the fn key with the f4 key as designated on my machine. I got a brrrrr sound but nothing happened, tried the fn & f4 keys several times and got the same result.

Played around with things and finally got the laptop display to come on, but it will only come on with the cable for the external display attached, and it seems the laptop is somehow dependent on the extentla Dell monitor to be connected to have either one of the displays to work. The little monitor icon just above the touch pad remains lit (in orange) no matter what I try, probably indicating this dependency situation.

The display software is NVidia and only through the NVidia control panal was I able to get the laptop display to function. However, as stated before if I unplug the Dell cable the laptop display goes black.

Any ideas on what I need to do to restore this laptop to normal opertio9n?

Thanks for any advice!