Q: The car cannot be started randomly

My car occasionally cannot start up. It sometimes runs great while other times it cannot turn over for no obvious reason. It happens randomly and lasts for an unpredictable period of time. When I put the key into the ignition, the car would be unresonsive. If I try again, it can be turned on or not. A few months ago, it happened that it wouldn't start at all for several days. After that, the car had no issues whatsoever. This difficulty starting up happens if I drive all day or after the car sat for a whole day without driving. I had the battery checked. Also, no codes came up when the mechanic ran computer diagnistics. They say that everything is fine with the vehicle. What could be causing this? Thank you.