You can activate the light on the inside of the oven by pressing the oven mode button (the button that lets you set the functions). If you then switch on the oven by selecting a setting and temperature, the lights should work again.
If this does not solve the problem, we advise you to check the oven lamps for possible defects. If a lamp is defective, then the oven lamp will have to be replaced. Use a heat-resistant oven lamp, which you can obtain in any shop that sells light bulbs. To gain access to the lamp in the oven, you must first remove the lamp glass. As grease and dirt will have built up over time, we advise you to remove the lamp glass with a cloth. If unscrewing the lamp glass proves difficult, you can use tongue-and-groove pliers.