Is the appliance connected to power? Do you hear the smooth whir of a healthy fan? Does the door close and seal correctly or is there air entering the oven? Which internal elements are heating up successfully and which are unresponsive? Are your normal oven settings on or have you recently been fiddling around with dials? Try to narrow down what may be affecting the functionality of the appliance.
Frustrating but true: many functionality issues with ovens actually stem from poor cleaning and maintenance. Giving the unit a proper clean-out will ensure there are no pesky food residues causing difficulty - after all, it can be quite alarming to find smoke billowing out of your oven door! Likewise, sometimes greasing a hinge or replacing a simple grill element can be a DIY job. It all depends on your confidence and previous experience.
In some cases, especially if you’re not 100% sure of the origin of the fault, the safest option is to call in some professional assistance. When it comes to diagnosing and dealing with complex internal faults, you can’t go past the expertise and experience that comes with having tackled such problems in the past.