This happened to my car also for no explainable reason.
Disconnect the battery and loosen the wires on the starter and jiggle them.
REMEMBER to DISCONNECT the battery.
It also happened to me again several years later and then all of a sudden it
would start. I have more grounds than came from the factory and all the
connections are clean and tight. It was like a Twighlight Zone episode.
I never did solve the problem and several 'pro' mechanics who I know also
couldn't figure it out either. Never happened again since that time several