If the hot tub has no power, or is not heating:
Hot Spring Spas: Place a droplight with a 60 or 75-watt bulb inside the access panel over pump one. (Facing control panel, usually behind right wooden section)
Sundance Spas: Place a small, UL listed, space heater into the main equipment area on LOW HEAT, HIGH FAN.
In both cases, make sure neither the drop light or space heater is not touching any components, wires, or insulation and replace the cabinet panel to the best of your ability. Leave the hot tub cover on and fastened to retain as much heat as possible.