You've lost your drivers. I assume you are using windows - so with the phone plugged into pc -> go into pc device manger and uninstall any/all drivers that show android device - unplug phone -> reboot pc and then plug phone back into pc. you should see the drivers reinstalling - see if that works.
edit: if above don't work try this http://androidxda.net/download-install-google-nexus-6-usb-drivers-for-adb-fastboot/
Still nothing. While connected via fastboot, my PC will only recognize it as an unknown device. Then says the correct drivers are already installed for an unknown device. It's not a driver issue. I think it's some bug from when I updated to 5.0.1. Or hardware. Which is why I'm trying to update to 5.1 without a PC. Hopefully this will fix the issue. I'm also looking for a way to completely unroot and go back to factory settings without a PC to get the OTA.