Normally it is just a case of the bulb failing. Fit a new bulb.
Swap it over to the other side to make sure it's not another faulty bulb.
If not, check the fuses (
Check the bulb connector with a multimeter. There is a chance that with
the headlights on, you will still get a reading of approximately 12 volts.
Quite a few smarts suffer from burnt connections at the fuse box. A poor connection due to
over wattage bulbs or poor quality bulbs causes an increase in arcing at the fuse box connector.
This arcing causes heat which damages the connection and increases electrical resistance.
Voltage can usually get through but there isn't enough contact to allow
enough amperage through to actually light the bulb sucessfully.
Drop the SAM unit down and check the nearest connector. N11-3, it's brown.
Look for charring, burning, overheating or any damage. Thanks to Piers M for the photo.
The 4 wires that feed the lights are shown below.
1 - Grey/Red - Left Low Beam
2 - Blue/Green - Right Low beam
3 - White - Left High beam
4 - White/Blue - Right High beam
1 - Grey/Yellow - Left Low Beam
2 - Grey/Violet - Right Low beam
3 - White/Black - Left High beam
4 - White/Red - Right High beam