It could be out of oil.
It could have lost its prime
It could be a defective check valve in the pump.
It could be the oil pump itself.
It could be bad electrodes.
It could be a plugged oil filter.
It could be a defective cad cell detector.
It could be a defective nozzle
It could be a bad transformer.
It could be a bad motor to pump coupling.
It could be a bad reset safety. The one you keep pushing the button on.
It could be the electric motor for the burner.
And on and on.
There is no such thing as a likely cause since all the above problems can stop your burner from working.
Educated guess? Primary control since it will not reset. Some of the newer one have a anti tamper design.
Time to call a pro for ON SITE repair. http://customer.honeywell.com/techli...it/69-0618.pdf http://customer.honeywell.com/techli...t/69-0617B.pdf