When you pull your brake lever, it in turn pulls a cable that ultimately squeezes your brake pads - forcing them to make contact with your rim (see image below). You should first ensure the cable exists and is properly attached. Sometimes, the attachment bolt comes loose and the cable slides freely (your symptoms indicate such)
When you release your brake lever, springs on the brake itself pulls back on the cable, causing your lever to return to its position.
If you disconnect the cable shown in the image and squeeze your brake by hand (at the pads), it should return to its open position when released. If you pull on the cable with one hand and squeeze the brake lever with the other, you should feel the cable being pulled out of your hand holding it. Pulling on the cable should return the brake lever to its original (non squeezed) position.
You can disconnect the working break to verify what 'good' feels like. Armed with this knowledge and a little bit of curiosity, you should be able to get to the root of your problem.