Check the timer is on the 'manual' setting
Some ovens will have a clock or timer function that allows the oven to come on later in the day or switch off after a period of time. These type of ovens need to be set to the manual mode to allow the oven to start cooking straight away. To check that the oven is set to manual please refer to the instruction book for your model. If you have lost the book you can download a copy from the website.