As Norm has said it’s 9 times out of ten the kill switch.
It’s very likely just that the rubber insulator has broken down which is simple to fix, either that or the insulation on one or both the kill switch wires has worn off.
It’s a five minute job if the first; you replace the insulator but you need to get one, still available from most decent suppliers. It’s a black boot thing about the size of the end of a kids finger. One wire goes in the side the other in the end.
Ten minutes if the second. You pull both wires, clean the terminals and resolder before sliding on some heat shrink insulation to replace the worn bit(s) of insulation.
Other things are possible but much less probable.
