Although Mag-Lite(R) is one of the most respected brands of flashlight, these flashlights sometimes need to be repaired. Since Mag-Lites(R) have survived being lost at the bottom of a lake or being run over by an 18-wheeler, there isn't much that can go wrong though. Anytime a Mag-Lite(R) doesn't work, there are two possibilities you should troubleshoot. First, make sure the batteries are in good working condition. Second, make sure the bulb is in good condition and electricity is flowing properly to the bulb.
Remove the end cap and take the batteries out of the Mag-Lite(R) handle.
Check the interior for moisture and check the O-ring on the end cap for damage. If there is moisture inside or the O-ring is damaged, replace it.
Install the new batteries and replace the end cap. Test the flashlight. If it still doesn't work, proceed to troubleshooting the bulb.
Remove the lens cover and the reflector cone. Remove the flashlight head and examine the O-ring beneath it. If it is damaged or if there is moisture inside the head, replace the O-ring.
Remove the bulb assembly collar and the bulb. Examine the bulb for corrosion or debris that may be interfering with the bulb's operation. If it is clean, look for damage to the bulb or receptacle. If the receptacle is damaged, you will most likely have to replace the flashlight. If not, install a new bulb.
Reassemble the flashlight and test it. If it still does not work, the problem may be in the switch. If this is the case, you will most likely have to replace the flashlight, as the switch is sealed into the handle and replacement switches aren't usually readily available.