Def Leppard Guitar

I wouldn't say its a wiring issue. If it worked for 4 years, it should have been wired correctly. It is possible, but unlikely that a wire fell off inside, but thats rare, especially for only 1 pickup. More than likely your pickup selector is getting dirty or worn. When you have issues like this, its normally on its way out. You can get some electronics cleaner and spray it inside the selector and move it back and forth to help clean out the dirt and/or corrosion IF thats the case. The best way to diagnose this problem is with a multi meter that checks for continuity as well. You can get them at Home Depot/Lowes and they are a good investment for around $30. Anyway, I would start with the switch and the wiring on the switch first.