Being a 125 they could have saved money by linking front and rear in series. Therefore, just like cheap Xmas tree lights, if one goes out, both go out! Just replacing a bulb might cure it. If not can you hear the relay 'clicking' when using the non-working side? If yes, probably a wiring problem between relay and both lights. If no, then the coil in the relay could have burnt out, or there is a wiring problem between the switch and the relay. I would imagine that it has a fairly simple wiring loom so it should be relatively easy to identify problems*. A multimeter is a cheap, and very useful, piece of kit to have in this sort of situation.
BB
*I should add a rider to this, in that whenever I do job that appear "fairly simple" they often take for ever!!!!!