If you have an electric Cooker or Fan Oven which is not heating it could be one of the following problems.
1, If the fan in the back of the oven is going round and the temperature light is on, but there is no heat then you will probably need a new oven element, these are generally very easy to fit and readily available on Ebay.
To gain access to the element you will have to remove the backplate from inside the oven. Once removed you will see the fan and element, if you look closely at the element you will probably see a small hole burnt in it where it has blown. Sometimes however there is no evidence of a fault but the element is open circuit ie: broken inside the element casing, either way it will need replacing. Pictures below of more common fan oven elements.
2, If your fan oven is getting warm but takes a very long time to cook anything, then you need to check if your oven fan is working. As the element is heating but the fan is not blowing the heat into the oven. To replace the oven fan motor is a little more expensive and a bit more involved.
If you have the opposite problem ie: your oven is over heating and burning everything then you will need to replace the oven thermostat
3, This one is quite common and easily overlooked if your oven is not working at all ie: no heat and no fan then the chances are that someone has put the oven into AUTO mode, simply put oven back to MANUAL mode and all should be working again.