Try yet another bulb. I've replaced a bad bulb with another bad bulb before because I packed away a bad bulb instead of throwing it away. If this still does not work then it could be that your switch has gone bad. I would then call MagLite's customer service and they will probably send you a new switch assembly.
Check for battery voltage between the bulb center connector on the switch and the body. Clean and lightly lubricate the connections. Test the bulb either in another flashlight or check it for continuity with a multimeter. Make sure your tailcap is screwed down as tight as possible. You might remove the switch and check it for continuity. New switches are available.