Metal halide lamps belong to a family of high intensity discharge (HID) lamps that includes mercury-vapor and sodium-vapor lamps. Like fluorescent lamps, HID lamps produce light by the excitation of a gas confined between a pair of electrodes and require a ballast to limit current draw and to provide proper starting and operating voltages. If your metal halide lamp does not start or burns irregularly, there may be a problem with the ballast or the bulb. Metal halide bulbs are long lasting, but they do eventually wear out.
Check the lamp -- it may have become loose in the socket. If that's the case, tighten it. If you suspect the lamp may be blown or defective, try a replacement lamp.
Read the ballast specifications on the nameplate, including voltage, wattage and lamp type, and be sure they agree with the characteristics of the bulb.
Turn off the breaker supplying the lamp and tighten the connections on the ballast with a screwdriver. If the lamp has never been operational, make sure the wiring agrees with the diagram on the ballast label. Be sure the voltage supplied to the ballast is correct. Some ballasts require 240-volt power, supplied by two hot wires carrying 120 volts each and connected to two separate breakers in the panel.
Look inside the ballast for a blown fuse and replace it with an intact fuse. Check the ballast output with a voltage tester. Turn on the lamp and touch the ends of the tester leads to the lamp electrodes with the bulb in place, then remove the bulb and make the same test. You should get voltage readings in both cases that agree with the output specified on the label.
Disconnect or turn off all other electrical components that are on the same circuit as the lamp and then try the lamp again. If it turns on, the circuit is probably overloaded.