I had the same problem with my LG monitor and an Nvidia graphics card from Dell. I had to run a Dell recovery file from a boot CD. There is a list of instructions that I got from this link. And I found the Dell Recover file at this link. I followed the directions and it worked.
On a side note, the directions say to:
Shut down the computer.
Disconnect the power cables from both the monitor and the computer.
Disconnect the VGA cable, and reconnect the monitor to the computer via the DVI cable.
Press the power button for both the monitor and computer while unplugged to drain any remaining power.
Re-attach the power cables to both the monitor and the computer.
This process alone may have fixed the problem, but I ran the boot CD anyway to be safe. Try it and see.
Thanks tons for this. My LG E2351 LED 23" external display for my macbook pro 17" was refusing to run at any resolution higher than 1440x900. I had somehow gotten it to 1920x1080 but when I unplugged my laptop, found myself stumped for 5 hours. Every time I would set display to 1920x1080, I would get the good old "D-SUB POWER SAVING MODE" message on the display. I tried turtle shell sleep / wake method and got it back briefly but then lost it again when laptop was opened. The directions that you posted got it straightened out on the first pass. Brought my gear from the US to Africa, and not like I can take it back to best buy or go shopping for different cables :)