It sounds like you have a standing pilot light. The burner won't light unless the pilot is lit. The pilot won't stay lit unless the sensor that sits in its flame says it is lit (i.e., it gets hot, and sends a signal to the gas valve). So, first, make sure that the sensor is in the flame of the pilot. It might also be dirty, so clean it off, or reposition it if required so it is in the pilot flame. If that doesn't work, if you have a multimeter and the manual, you can test the sensor (thermocouple) to see if it is within specs. If not, you'll have to replace it.