Most common failure of the halogen-lamps not working is the lamp-holders themselves. --They don't normally ALL fail together however, so I suspect summit else....
Ive not had a Toroidal 12V Lamp transformer fail in one of these--YET!--but have had the connection-block to the lamp-holder tails fail.
--IF there's Mains Input TO the lamp Transformer--There'll be power coming Out of it--OR its faulty, however, there's a relay between the Transformer and the mains (on mains side of Tx) and this relay is positioned on and controlled by the control-board.
Another fault that's presenting at the age these fires are at is the LED 12V DC Power-Supply that the control-board and mist-maker are supplied by--When this goes, There's no operation of the fire, lights dark and no mist, or crazy flashing on/off of LED's in logs and beeping of controller when you try and turn the effect on.
--This should produce 12V DC to the Control-Board at all times the Mains-Switch on the fire is on, and also when the fire effect/lights/smoke etc is selected.
--They are only £8.50 so worth changing anyway. Be sure its a DC 12V LED supply rated at 30W. --Ive used these quite successfully-
They are fractionally longer than the original, but still fit. They are marked for voltage and polarity just like the original easy to changeover and are equivalent to the original.
NEVER use the more usual 12V Halogen type lighting supply to replace it with, as these are AC and will destroy the control-board and mist-maker.
These Opti-Myst fires are a great idea--Very Crappily made, with cheapo components. Dimplex don't help as they are profiteering on the cost of the idea--And the spares, IMHO.