O/D light blinking

The O/D light on the dash board started blinking and the speedometer is not working, a friend of mine tried to check if it was a sensor on the transmission and he went under the car. I really do not know what he did but the transmission feels heavy sort of forced when I accelerate and that was not how it felt before he went under the car to check the sensors on the transmission. I want to know if by connecting and disconnecting something under the car near the transmission could have done harm to my transmission? Is it safe for me to drive the car?