If this is a power vented unit, it is likely there is either a blockage in the intake or exhaust line, or a broken piece. When the burner comes on, the exhaust fan comes on to move the exhaust. There is a pressure switch safety circuit that detects the fan is on. If that switch doesn't close, it will shut the system down. It could be a bad switch, bad fan, or an obstruction or broken pipe somewhere, preventing the pressure to build up as a result of the fan motor running. It could even be the fan blades may have broken, and can't move enough air to close the switch. Checking the switch should be easy if you have and understand how to use a multimeter.