If you haven't done it already, go to Device Manager and see if the system sees a CD/DVD player. If it does, uninstall it and reboot. This should force the system to re-install the drivers. If it still is not recognised, then it may be that the connections to the player are loose. With the computer turned off, open the case, earth yourself and re-seat the connections to the player. That includes the power supply AND the data cable. Reboot again and if it's still not seen, then you will most likely have a dud player. Annoyingly coincidental since you've just attached a new monitor.