1999 Ford Expedition 1999 Expedition picky about starting

Truck showed no signs of a problem, stopped at a mini mart, when I turned the key to start dash lights came on but engine wouldn't start. There was no cranking or clicking, nothing. Radio, windows, dash lights worked fine. Came back a few hours later, started fine and ran fine. Checked all fluids, fine. Four days later, same exact thing happened. Waited with car for an hour, turning key every 15 minutes. Started exactly one hour later. Could it be the starter solenoid or the starter itself. Or am I looking at something with the ignition switch. I am a single female, can add/check fluids, change tires and headlights not much else beyond that but a friend is willing to help.