The fact that a serial port no longer appears implies that the arduino board is no longer working.
Have you disconnected the servos from it? There's no way you're going to supply power for 6 servos from a computer's usb port (they may be trying to pull enough current at startup that your computer is turning off power to the port, either by detecting the overcurrent condition and turning off power, or via something like a self-resetting PTC fuse)