This is unfortunately a problem found in all Nokia wired headsets.
The connection to the speaker in the bud is very weak and they often fail, leaving a dead side.
Although it is possible to repair, it is a task that requires very uncommon tools and a high competency in soldering circuitry, best left to those who are highly skilled. If it is not something you are able to undertake, the cost of having someone do it for you outweighs the cost of replacing the headset. =o(