It's about as basic as it can get, if your oven isn't heating, getting to the correct temperature, and staying stable at the setting you dialed, you either won't be able to cook at all or not very well. Things that can stop your oven from heating properly are slightly different depending on if the appliance is gas or electric. Here are the most common reasons for problems with each type.
Four Reasons Why Your Electric Oven isn't Heating
No power to the appliance
Most electric ovens have a clock these days and a quick glance at that will tell you if power is getting to the oven. If it's not, check your circuit box, and reset the trip switch if required.
A tripped switch or blown fuse within the oven
Some ovens have a circuit breaker with a reset button on the control console or on top of the oven, it's worth checking your owners manual to see if your does. If resetting the trip switch doesn't solve the problem, make sure the oven is cut off from the mains before attempting to look for blown fuses within the appliance. You may find fuses under the range top, in the back of the oven space, or behind the oven.
Pulling the oven away from the wall to check for fuses at the back will also allow you to check that the plug is still properly engaged in the socket if your oven is a plug-in variety.
A broken element
If the oven is getting power but isn't heating it's possible that one of the elements has burnt out. This is simple to check, turn on the oven, set it to hear and inspect the element to see if it's glowing evenly. No glow or a patchy color indicates an element that needs to be replaced. Unless you're very confident about your handyman skills it's probably time to call a technician to arrange for oven repairs.
Thermostat replacement is complex enough to need a professional but you can certainly check your thermostat with an oven thermometer. Set the oven at 300, 400 and 500 degrees, checking the thermometer at each stage. An error of more than 50 degrees is a sign that the thermostat needs replacing.
Three Reasons Why Your Gas Oven isn't Heating
Most gas oven repairs should be done by a professional, but so long as you're aware of basic safely precautions there's no reason why you can't try to locate the problem.
The igniter does two things, it draws gas through the safely value and heats up to ignite it, either of these aspects can fail. A weakened igniter won't open the valve and if there's no power to the igniter it won't heat up. Check for a weak igniter by eye, if you can see that it's glowing but the gas is not lighting it probably means the valve isn't opening. If you can't see a glow the igniter needs to be tested to see if it's getting a current. This is done with a multi-tester.
A failed safety valve
It's possible for the valve itself to fail if this happens gas will not flow into the oven so there's nothing for the ignition to ignite. The
Check for this in exactly the same way as you do for an electric oven - with a thermometer, at different temperature settings. Replacing the thermostat is a job for a qualified oven repairs technician.