Check your throttle pressure linkage. See if something has changed (is the spring still pulling it back to the minimal position).Also try jacking the rear wheels off the ground and running it through the gears,
(be careful) it is easier to make one upshift with the wheels off the ground. If it still does not shift than something has happened to the govenor making the pressures off. maybe drop the pan is the next step. Post what you find out .
