Hello tnhnt, it could be something as simple as a bad trigger switch/connection or possibly a bad ground clamp connection, meaning the pigtail ground that makes the connection with the machine ground. If you have reasonable knowledge of continuity and using a multi-meter try using the continuity function of the meter and check for proper operation of the trigger switch on the gun. To do this, disconnect the plug from the face of the LN 25 and check for continuity accross the two terminals of this plug when you depress the trigger switch. If you don't have continuity when you do this then the problem is with your trigger switch.
     To check the ground pigtail I would suggest taking the cover(s) apart to the point that you can access the end of this wire where it connects within the feeder as well as the end where the clamp is attached. Do a continuity check on this wire by attaching one of the meter probes to the clamp and the other probe to the other end of the wire. If you don't have continuity here you should probably either replace the entire wire or determine if the clamp has a bad connection or if there is an internal break in the wire itself.
     There is also a small possibility that the polarity switch that is on the face of the machine could have an issue. This switch is the one that says that it needs to be set to the same polarity as the LN 25 is hooked up to the power source. If this switch is bad it could prevent the circuit from being completed that powers up the feeder.
     There are likely some other basic checks that you could perform but I can't think of them right now, maybe some of the other folks would like to add more to this. Good luck on your problem resolution. Best regards, Allan 

If the contactor isn't pulling in, another thing to check would be the coil of the contactor. It should have two small wires going to it. You can check with an ohmeter with one of the wires removed, there should be like less than 1000 ohms. Or with ohmeter set on X10 you should see at least some small movement of the needle.

Another way to check the contactor coil is with a dc voltmeter set to check for about 75 volts dc, hook the voltmeter to the two small wires on the contactor coil and with the machine set and ready to weld, pull the trigger. If you have between 20 and 75 volts dc but the contactor isn't pulling in, you need a new contactor.

Sometimes on the LN25 the contactor will become stuck, full of dirt or old pieces of welding wire or other debris. check to make sure the contactor operates freely.