For a gas wall oven that isn't heating, check your gas supply cut-off valve to make sure it's open. A weak or broken igniter can also prevent a gas wall oven from heating. A broken igniter won't glow. A weak igniter will glow but it won't get hot enough to ignite the gas, so the safety gas valve won't open and the oven won't heat. Replace the igniter if it's weak or broken.
In an electric oven, a broken bake or broiler element can prevent the oven from heating. A tripped thermal switch on the back of the oven can also prevent an electric oven from heating. Reset the thermal switch if it tripped. Electric wall ovens require 240 volts of electric power to heat, so one of the two 120-volt house circuit breakers can trip, preventing the wall oven from heating. Check the house circuit breakers for the wall oven and reset a breaker if you find one tripped.
In either a gas or an electric oven with dial controls, a failed oven control thermostat can prevent the oven from heating. For electronically-controlled gas or electric wall ovens that have a digital display, a failed keypad, bad electronic control board or faulty relay control board can prevent the oven from heating.