one side of cantilever brake pads does not spring back Same as this question one side of brake pad retention problem (i.e. attached to rim) but mine is a cantilever brake. The answer there doesn't work as mine doesn't have those screws and the brake holders look very different.

The bike is a mountain bike if it matters.
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.