Altium view does not show lower part of PCB
It sounds to me like you have your PCB located at or near the absolute origin. The origin you can move is called a "relative origin" and moving it does not change the absolute origin. Before starting a design, I always move my relative origin far away from the absolute and build my board around that, rather than down in the bottom left corner of the workspace. This allows full navigation of the entire board without running into the absolute origin. Before I started doing this I had the same problem you are seeing. To confirm whether or not your board is well within the workspace, watch the coordinates at the bottom of the screen as you mouse over or near the "frozen" part of your board. If I am correct, the Y coordinate (and probably the X coordinate as well) will be low or zero. I'm not sure if it would show negative numbers.

UPDATE:
After testing this out at work it looks like the coordinates at the bottom of the screen are based off of your relative origin, so not necessarily of much use here.  However, as your cursor reaches the absolute origin you will notice the numbers stop changing as you continue to move down and left. That will help you pinpoint its location, and give you an idea where you need to stay away from.