UPDATE (5/4/16): It looks like the updates are finished and the server list should now be populating correctly for most players. Check that Source is set to Internet, then click on Refresh List from the top left corner.
COD 4 servers aren't displaying on the Master List.
Random maintenance can occur causing the list of servers to no longer display temporarily. When this occurs, you can manually connect to servers until the maintenance is completed.
1. Select a server from this link: http://www.gametracker.com/search/cod4/
2. In the COD 4 MP Options menu, set Enable Console to Yes.
3. Press the '~' key on your keyboard to display the console command line.
4. Type in connect (IP NUMBER FROM GAMETRACKER LIST) then press the ENTER KEY.
The process is demonstrated here: