Oil Boiler 'Lock Out' Light?

I have a Thermecon oil boiler which for the most part has been trouble free. I've only had it professionally serviced once and I figured that anyone can sweep and vacuum out the fire box and change the jet! So for the past 12 years thats been fine. This year in addition to the jet I changed the Blast tube as well. I ought to say here that the reason for even looking at the boiler and subsequently changing both jet and blast tube is that the 'Lock out' light had come on. Assuming the thorough cleaning etc. would cure it, it came as rather a suprise when after 2/3 hours the lock out light came. It wouldn't immediately re-ignite but after 2 hours it ran for a further hour before locking out. Anybody out there that knows why, I'd be glad to hear from them.
Many thanks, Harry