fastboot devices gives one line ..
"FS-bunch of numbers fastboot"
adb devices lists no devices
We're at fastboot, ( should see fastboot (usb) ) so only the fastboot command is interesting. So, on the "fastboot devices" command, which is what we *really* want right now and I think you're talking!
So, just to be sure, the 'bunch of numbers' should be your device serial id. That we see that means we're talking to the phone finally, and we have fastboot working, I believe.
Ok, so @rootlinux
was suggesting the possibility TRWP is bad. The question is, can we reflash TWRP and get a good recovery. So to test that first, let's see if fastboot works at all -- try to "fastboot reboot-bootloader" and see if that takes us back into the bootloader.
After that, I'd think we come back to fastboot (usb) and see if we can reboot to RUU "fastboot oem rebootRUU" but don't run to that yet.
I'm hoping @rootlinux
chimes in -- he had just this scenario and should have good insight right now.