The oven not turning on or heating properly are common oven problems. The defective oven parts causing these issues are usually a defective igniter and damaged bake element.
The igniter is responsible for opening the oven safety valve by drawing an electrical current through it. Its second function is igniting the gas in the oven burner. If the safety valve does not open, the oven does not heat.
The bake element directly heats the oven. The bake element glows red hot when it's working. If it does not glow red hot, it is likely burned out and needs to be replaced.
A multimeter is used to test the igniter and bake element's continuity. If they are determined by the multimeter to not have continuity, they are no longer working and need to be replaced.