I changed the spark plugs out in my envoy one at a time because it has individual coil packs on it and I dint want to get them crossed up . After changing the plugs I tried to start my truck and it would not start. I then checked each coil pack to make sure the connection was good and tried to start again, still wouldn't start. I then put the old plugs back I to make sure the plugs that I had purchased was not a bad set of plugs and it still will not start. What will cause that to happen.