Toaster oven knobs vary considerably, but they can generally be pulled away from the unit and removed, which might reveal the problem. If the metal part the knob connects to can be turned, a simple replacement might fix the problem.
Oven knobs are generally cheap to purchase online, and the store from where the oven was purchased might also have replacement parts in stock. If the metal part under the knob doesn't turn or doesn't change the temperature of the oven, the unit likely needs to be disassembled to be fixed. Although some problems might require replacements, repair experts might be able to fix the unit in a short period of time.