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.