Welders that won't feed welding wire typically have one of two things causing the problem. Either the welding tip has built up with slag, or the wire feed tension spring is loose. Damaged internal motors should not be repaired at home. It is best to have internal electrical problems fixed by a qualified welder repair technician.
Unscrew the tip from the welding gun. You may not be able to tell by looking at it if the tip has become clogged, but tips are inexpensive, so replacing them is only a minor inconvenience.
Pull the welding wire through the welding gun, and then insert the welding tip over the wire. Screw the tip into position, and then try the welder to see if the wire feeds. If not, continue to the next step.
Open the side of the welding unit to check the spring tension on the wire feed unit. This is controlled by a wing nut that holds an arm down, squeezing the welding wire between two metal wheels. The lower metal wheel is the one that is under power. Tighten the wing nut down slightly, but not too much. Too much tension could crimp the welding wire.
Actuate the trigger of the welder again, paying particular attention to the lower wire feed wheel. If the wire feeds, the problem is resolved. If you see that the lower feed wheel is not turning, remove all tension from the wing nut and try again. If the wheel still does not turn, the wire feed motor or circuit board has burned out and needs replacement by a qualified repair technician.