When opening a world, if the nether portal spans more than one chunk, the portal frame generates, but the nether interior only generates in one of the chunks.  In order to fix the portal must be broken and reset.  The portion of the portal that is not lit will not transport the player to another dimension, but the portion that is lit still works as normal.

Additional note - if you try to relight the portion that is not active the obsidian will catch fire and the active portion of the portal will go out.  At this point you can put the fire out and relight the portal.