1995 Buick LeSabre trans help

The transmission relies on pressure. If the fluid is low then you have low pressure which will not engage the torque converter. If the fluid is fine then it could be as simple as a service to change out the transmission filter which is clogged and can restore the pressure. If not that then the TCC solenoid is faulty and not supplying the torque converter with the proper pressure to shift like it should.