Ok, since unlocking the bootloader worked we can assume the cable is a working data cable (you could try a different one to make sure it didn’t break just now).
Maybe there’s driver trouble. Here’s a generic driver and ADB/Fastboot repair …
- Disconnect the phone from the computer.
- In Control Panel - Programs and Features:
Uninstall the Android SDK (if installed) and everything with “MTP” or “ADB” in its name. - Reboot Windows.
- In Control Panel - Device Manager:
Enable View - Show hidden devices, and then uninstall every possible ADB, MTP and smartphone device, hidden or not. - Reboot Windows.
- Connect the phone again. (Windows should install the necessary drivers then automatically. They can be named after a different device, this doesn’t matter.)
- Download the current Android SDK Platform Tools
- Unzip the ZIP file to e.g. C:\platform-tools (you can unzip it to wherever you like).
- Add the path to where you unzipped the tools to the Windows “PATH” environment variable, so adb and fastboot could be run from everywhere without always having to prefix them with the path.
Then try fastboot devices again.