If it stays in 2nd gear, it has defaulted to " Limp-In" mode. It could be a sensor problem, but with the mileage, it's way past the point where slipping clutch plates occur. Turn the ignition switch off, then restart the engine to get it out of limp-in mode. If it starts out right away in 2nd gear, it's most likely a sensor issue. If it starts out in 1st gear once, then shifts to 2nd and stays there, it could be worn clutch plates meaning it's time for a transmission rebuild.
There are a lot of updated parts available to transmission shops that greatly increase the reliability of the rebuilt unit.