Why is my Xbox controller connected to Xbox, but not working and not showing as connected?

I have been trying to troubleshoot this for a while and the only conclusion I can come to is that my controller is defective, or Microsoft has added an update that made it stop operating correctly conveniently within days of the warranty expiring. Whatever it may be, here is the issue:

Controller suddenly stopped controlling the device. It was being used by second player when their screen stated that the controller disconnected. I attempted to sync the controller with XBox again and it never comes up as successfully syncing. I turned the controller off and back on, tried again. Nothing. I removed the batteries and replaced them with new batteries and tried again. Once again, the device did not sync. However, the Xbox button on the controller would occasionally turn solid again, but still not control the XBox. The controller is for some reason not listed under accessories next to the other controller. We tried connecting the controller via USB to the xbox and had no luck getting it to come up. There is no possible way to update the firmware on the controller. It is not recognized by any device. I have tried to pair it with my computer, but upon turning the controller on, it will turn on the XBox. XBox, again, does not recognize this controller as a device that is synced. Once the XBox turns off, so does the controller.

I am completely stumped and absolutely livid. 3 days past the warranty and somehow, some way, the controller suddenly doesn't work due to nothing at all happening. No dropping, no debris in the controller, it is as clean as it was 7 months ago upon purchase. I am 1000% positive this controller is either defective, or Microsoft has processed an update to make it defective.

Thanks.

:)