There are many reasons that the piolet light wil not relight - check them in order.
Is the system pressured (usually over 1 bar - less will not relight)
Thermocouple failure (replacement use universal termocouple (£6))
Is it powered and fuelled (gas and electric)
System set to 'off' (try hot water+heating, timer set to manual override ON, maximum temperature if adjustable, maximum thermostat) then turn on the electric and wait 30 seconds before trying the igniter.
Steve, February 2008