one side of cantilever brake pads does not spring back
If you have cantilever brakes with no adjustment screw you can usually bend the spring. 

At the top of the arm the metal spring probably pushes against a post. On the side that's stuck to the rim unhook the spring and push the spring away from the rim. Don't do it too hard to start with. Hook it back to the post and try to see if it's balanced the brakes. Keep trying until the brakes balance.