The ‘Device state: locked’ message means your bootloader is locked, which is the factory setting - just so we’re clear it has nothing to do with your issue.
But seeing as it’s booting to the bootloader, it really ought to show up in fastboot devices.
Are you using Windows? That operating system is known to be a bit particular when it comes to using the adb. First thing to try is using different USB ports.
If that doesn’t work:
So essentially, removing the device from your computer’s memory (the Device Manager) so it makes a fresh record when you reconnect the phone.
Oh, and this is going to sound stupid, but I’ve lost a lot of time on this sort of stuff myself: are you sure the cable is a data cable and not just a charging cable? Quickest way to check is to hook something else up to the cable and see if it shows up as a USB device.