I am a college student and don’t know much about bikes. I rode my bike to the store and locked it up. When I tried to leave the back wheel wouldn’t spin and the pedals wouldn’t move forward. I called my stepdad and he said the chain probably moved off the wrong sprocket (I don’t know what any of this means) but I ended up moving the chain to the biggest sprocket of three by the pedal. Still wouldn’t go. What I’ve found is that the pedals won’t move forward while on the ground. The pedals can go backwards whether it’s off the ground or not. When I lift the bike off the ground by the seat, the pedals will go forward. I cannot post any photos since I am new to this page.