Nexus 5x stuck on Google logo
Get into Recovery, if you flashed a CM13 you probably have a custom recovery installed. Do a factory reset from your custom rom. Alternatively, you can wipe Cache, Data, System individually. 

After this, try rebooting to bootloader and connect your phone to a different PC if your's doesn't recognise the phone. It is preferable if the PC has Linux as it doesn't get you in device driver hassles. If anyway you have no option other than a Windows system, install universal naked drivers if your own device's official drivers doesn't work. Reboot your system and connect your device again to try getting Fastboot mode.