The pan and/or zoom tools have become disabled in the Tools bar.
If the data frame is set to fixed scale, the zoom tools become disabled as their use would change the scale of the data frame. The pan tool, however, remains enabled. If the data frame is set to fixed extent, both the zoom tools and pan tool are disabled. This is by design.
To regain use of the pan and zoom tools, set the extent of the data frame to automatic.