Could be one of a number of things,
Blocked APS Tube, Faulty APS, Electrode or Lead faulty, Gas Valve not opening, NO GAS (have you a CARD METER, ARE YOU OUT OF CREDIT?) OR pcb
iF YOU HAVE GOT A SUPPLY OF GAS, YOU WILL NEED A REGISTERED ENGINEER
www.gassaferegister.co.uk will assist you to find a a Qualified person near to you
www.elanar.co.uk, September 2009