Hi there, check your post at BHM
Probably a problem with different refresh rates with you new monitor.
Try this, open cfg.txt in your lfs folder and change [Start Windowed 0] to [Start Windowed 1] and try starting lfs again. This wil set your LFS to startup windowed. That wil enable you to reselect your refresh rate.
If this doesn't work, set [First Time] to 1 or delete the whole cfg.txt.