Does it not have a separate standard halogen bulb for the main beam? If it does, then check the earth inside the headlamp unit.

I had a similar problem not so long ago on my Megane. I replaced the bulb (tried 2 different bulbs) - no luck. Then checked the fuses (each side has it own fuse), still no luck. Traced it to the earth inside the headlamp unit having come off. Clipped this back on (all by feel as you can't really see what you're doing) and all has been fine since.

 

I have a hunch that the Lag 3 is Bi-Xenon (i.e. main beam and dipped beam are from the same xenon bulb/projector assembly and the 55w halogen is just for 'flashing'.) If that is the case, then the 'adjuster' which moves the assembly to create a different beam pattern is not working for some reason. It could be a mechanical or electrical fault, but a good first step would be just to clean the connections to the light (not just re-seat them, try some contact cleaner spray - handy stuff) and also inspect them to make sure no pins are bent (I don't know how this happens when they're plugged in but the terminals in my first Lag 2's back lights mysteriously mangled themselves in such a way as to kill the brake light on one side without my ever having disturbed the light - took a few minutes with the screwdriver straightening it all out :crazy It seems unlikely that a mechanism that's in regular use like that should seize up, but I guess it is possible too, or perhaps the servo is kaputt.