Our horizon dsx gate system suddenly los...

First of all check circuit breaker supplying the gate to see it has not tripped.
Also check power is coming out of the circuit breaker in case it is faulty.
You now know if supply is being sent to gate transformer.
Check supply is arriving at gate with mains tester. if so check low voltage side with small multimeter.ONLY DO THIS IF YOU ARE COMPETENT WITH ELECTRICS.If all power is reaching the pcb (printed circuit board) then a call out will be needed as the fault may be in the board itself.
here is a link to a brochere that may help.
http://www.sentexsystems.com/pdfdocs/6001037.pdf