Untick "hide modes that this monitor cannot display" in windows monitor settings. You can reach it by clicking the advance button on the screen resolution page.
If it's a monitor connected through D-SUB, you can as well try unchecking the "use EDID" option in CCC, somewhere in the analog display settings. After that, a reboot is required.
If you have a monitor connected through D-SUB (like before) you can as well install ATi Tray Tools and enter the advanced tweaks section, where you can enable the "CRT supports all modes" tweaks. A reboot is required for this as well.
If your monitor is connected through DVI/HDMI, using/not using monitor drivers can make a difference. I remember my old monitor went haywire if I installed its drivers on certain operating systems. I lost display modes just like you did now.
One of these (or maybe any of these) should work.
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64