According to your manual: Open the door. If the light does not illuminate, the bulb has probably blown. Call a qualified technician – do not try to change the bulb yourself.
Manual here: https://www.partmaster.co.uk/images/mediator/90/Kenwood%20K30CSS14%20IB.pdf
Having said that you can probably remove the cover and look yourself BUT BE CAREFUL. MICROWAVES CAN RETAIN LETHAL VOLTAGES - EVEN WHEN SWITCHED OFF! - If you didn't know that anyway you are probably best advised to ask a techie.
acp, July 2017
