I selected 2048xsomething today by misktake on Screen menu in LFS. Now whenever I open LFS the monitor goes black and comes up with the message "SIGNAL OVER RANGE". Please help!

Press shift-F4 after opening LFS, then go to the settings screen and change the resolution back.

Or alternatively, open LFS/cfg.txt and search for the line starting with "Screen Info" and replace the resolution values with something better, like for example "Screen Info 0 75 1024 768".

Your first method I can't use thanks for the 2nd!