(A) Please make certain that the black and white input wires (primary side) of the low voltage transformer are connected to the power line (120 volts or 277 volts) and that the two red output wires (secondary side) of the low voltage transformer are connected to the low voltage light source (12 volts or 24 volts). Most failures occur as a result of reverse or improper wiring.
(B) Check the filament of the lamp to see if is burned out. (Remember the glass envelope of a halogen lamp should NOT be touched by bare hands because the natural oil from your hands will cause the lamp to burn out prematurely.)
(C) Check the connection somewhere between the output wires of the transformer (red wires) and the lamp. The transformer has a sophisticated short circuit/overload protection system. If it senses a short or a bad connection or too many lamps (ie: too much wattage) it wil cause problems. Check all the connections for tightness, corrosion, arcing etc. If all are tight and clean and you do not have more than the maximum wattage of lamps on the system, then look at the lampholder itself. Make sure the contacts in the lampholder where the lamps plug in are still tight and do not show signs of carbon buildup or arcing.