im taking a slightly different angle here from adam, im thinking the light fitting is a fire rated downlight, which is good, ive made a few quid with this problem because its quick and easy to resolve, the fitting usually comes with a little suction cup to turn the lamps (lightbulbs) into the fitting, just press your thumb and forefinger against the lamp and push in and anti clockwise at the same time, then do the opposite to put the new one in , in and clockwise.
hopefully this is what your problem is and its now been resolved.
stoozybhoy, February 2010