Okay, then the easiest way would be to use a timer to time the period between pulses, and if that timer goes above some set count, it trips the circuit that lights the LED.
So the first part of the circuit would decrease the amplitude of the signal down to logic levels, and make a trigger signal from each pulse which was 18V to 24V in amplitude. That would be fed into the reset input of a free-running counter, so that the counter never reached the alarm count (which it would if a single pulse were missing, or some number of pulses in a row, depending on how you want to configure it). The alarm count would trigger a flip-flop, which would light the LED. The whole circuit can be reset with a single pushbutton, to reset the counter and turn off the LED.
Does that give you enough to build it, or do you want some more details on how you can make it? I'd start with a 74HC4060 counter IC, with maybe a 5MHz crystal on it, to give you a nominal count of about 5MHz/7.2kHz = 695, and you could use the 1024 count output to trigger the alarm LED flip-flop. Divide down your input pulses with a resistor divider, and run that into a comparator circuit that looks for the 18V minimum equivalent voltage level. The comparator output would feed the reset of the 74HC4060. The whole thing should be able to run off of a 9V battery for a fairly long time (use a low-current LED...).
