If you only took the old bulb out and replaced it,with the electrics OFF,do it again,they only fit comfortably one way round,you may have managed to fit it the wrong way round s**t happens,if you haven't disturbed anything and messed around you have to go back over your work,by the way the other bulb will pack up soon you may as well replace that,DON'T touch the glass with your fingers use a tissue or cloth.
Rich, January 2010