Exactly, starter spinning but engine won't engage. This is how engine blockade works, you spin it until battery runs flat, but nothing happens. Depending on the engine, ECU doesn't fire the spark, or shuts off the fuel pump etc, blocking the engine.
Keep the key clean, make sure there are no bulky metallic objects near ignition (key chain or something), keep away from strong radio wave sources... Still, it can happen again without any apparent reason.
This is the price we pay since our cars became so dependent on elecronics.
