if u already rebooted and still not showing, :
right click on My computer -> Properties -> Hardware -> Device Manager
on the console (window) that appears, select and expand DVD/CD-ROM drives
it should show your cd rom hardware if your OS is recognising it. if not right click on it and click 'Scan for hardware changes' (do this after reboot)
if it is showing, right click on it -> Properties
it shud tell u whether the device is workin properly or not. try selecting the trouble shooting option or right click as mentioned before and select 'Scan for hardware changes' and see if anything changes