Hopefully you can appreciate that with the very vague information you've provided in your question, I have no idea what brand or model of oven you have, so cannot give specific advice to better help you. However, the most probable casue of an oven not heating whilst the fan is still running is a failure of the heating element that surrounds the fan.
In most cases you will be able to remove a panel in the rear of the oven that covers the fan to expose the fan baldes and the circular heating element that surrounds the fan blade. Usually an easy part to source and replace. If you wish, use the Add Answer button above to provide the brand and model number of your oven and I may be able to provide more detailed advice to help you.
Rupert, December 2019
