As E DREAM says, you can manually operate the shift levers from underneath. First you need to ensure that the clutch is releasing. If the clutch is pressed to the floor can the engine idle?

Once under the car, wiggle the levers until they are all in the neutral position. The first thing to check is for tight linkage. The bushings on the trans levers or the ends at the shifter may be loose. If so, replace them all and properly adjust the shifter.

Good luck,