If the Hi-limit switch reset, and the heater came back on, the problem probably isn't the high-limit. The high limit is probably tripping (like it should) from excessive heat in the header. You can check the thermal by-pass, or the pressure/mechanical by-pass. You could also have an excessive build up of scale in the exchanger. If it has only happened once, it could be the scale. If left alone, you will probably experience the hi- limit begining to trip more often as the scale begins to build up more.