What was the last task you did before the door locked, was it a self clean?
Assuming you have checked its not on child lock, this sounds like a sensor fault, at which point you will need to get an engineer out to test the unit, this is not a consumer repairable fault.
If you would like me to suggest an engineer in your area, please provide your postcode
David if you can give me the model of AEG cooker I can provide you with the steps for removing the child luck, unfortunately the process varies dramatically depending on the model.
I would contact appliance repairs on 01592 802355, they are very reasonable and come highly recommended.