pilot lit on gas furnace, but the burners won't light
By far the most likely cause if you have a standing pilot (pilot on all the time) - is the thermocouple has died. It is a little probe that sticks into the pilot flame, and has a copper tube (or sometimes wire) back to the gas controller - if the thermocouple does not sense the heat of the pilot it prevents the gas controller from turning on the burners.
Biomass amp; Wood Fuelled Heating | Energy Saving Trust
2020-7-26 · Wood-fuelled heating systems, also called biomass systems, burn wood pellets, chips or logs to provide warmth in a single room or to power central heating and hot water boilers. A stove burns logs or pellets to heat a single room - and may be fitted with a back boiler to provide water heating as well. A boiler burns logs, pellets or chips, and is connected to a central heating and hot water