Your Guide has multiple "views." Some views include only a few channels.
While in the Guide, look at the picture in the top left corner showing the program playing on the currently selected channel. Below the bottom right corner of that picture you will see a word like "HD," "Subscribed," "Favorites," etc. indicating the current view. Pressing the Guide button again will change the current view.
Keep pressing Guide until you are on the desired view. "All" will show all available channels. "HD" will show only HD channels. "Subscribed" will show only channels to which you are subscribed. (For Expanded TV customers, "All" and "Subscribed" should be the same.)