Why isn't my car starting?

I was driving one day and the radio started flickering in and out and then the gauges jumped to max. I turned off the radio and the gauges went back to normal. When I returned home I noticed a ground that had come undone off the bolt to the thermostat housing. I reground the wire and went to turn over the car, and nothing. No click, no spin, nothing. The car still has power, I can turn on the headlights, the radio works, the interior lights and wipers work, everything. I took off the starter which resulted in a poorly spliced ignition wire to come undone. I hooked the battery up and newly repaired ignition cable to the starter and nothing. I took the starter to Advance Auto Parts and they tested it and everything came back good. What could be causing the starter to not turn at all with the turn of the key?