Neutral Safety Switch is a likely candidate. It permits the starter to operate only if the car is in park or neutral. They don't last forever. In the future, try moving the gearshift from park to neutral, then try starting. If that does it, replace the NSS. It's not expensive, and it's not difficult to replace.

Could also be the starter, corrosion on wiring connections, or even a bad battery. But NSS is a good place to begin looking.