Portal froze; game crashed; reloading hangs

Was able to use a Portal from over-world to Nether.  Blocked the portal to prevent mobs coming back.  Tried to return to the Over-world and the game hung up on the Generating Terrain screen.  After waiting, I ended the process and tried to reload the game.  The game appears to load my avatar in the portal and then hangs up on the Generating Terrain screen.