Before changing anything I would recommend that you start by installing a version of (probably OpenPilot) that matches the firmware you have inside the Revo.
Refreshing your knowledge with the old GCS and firmware and getting it flying like it did before is a good start to prove there are no hardware problems. Save your settings UAV file. Make some print screens of important pages like PIDs, etc. Then, armed with knowledge, you can consider upgrading to the latest LibrePilot for some new features.
Windows? What version? Can't connect is sometimes a driver issue. Others will probably post to point to driver issue fixes. I would try unplugging USB and use Device Manager to uninstall the <!> bad USB driver, and reinstall GCS, but I understand that may be overkill. Do the Revo LEDs blink the way you expect? What do the LEDs do?
V6? Maybe 16.09?
Hmmm. I remember a Terrier avatar from the OP days.