1) Not all LCD monitors can support 120 Hz refresh rates. Be sure that your monitor supports this high refresh rate. Also be sure that you have any applicable monitor drivers installed. The monitor drivers should unlock the ability to support the high refresh rate. If the monitor drivers do not unlock the higher refresh rate and you are sure that your monitor supports this refresh rate at the resolution you are using, in the NVIDIA Control Panel, under the Display section, select Manage Customer Resolutions and create the specific profile for monitor. Be sure to test your settings before applying them so as to prevent you from loosing all visual
2) Ensure you are using an NVIDIA GPU that supports dual-link DVI.
3) Make sure you are also using a dual-link DVI cable to connect from the LCD to your GPU. Your LCD manufacturer should have included this cable in the original factory box. The connector looks like the image below.