Just to confirm you are putting in a cd and not a record
Seriously though, this is the oddest question i've ever seen here. But lets see what we can do to help.
With this type of cd-rom you need to use a LITTLE
force on the clear plastic inner section of the cd to make it grip the spindle. Then the cd will sit flush with the bottom of the drive and you should be able to close the cd drive.
Of course this is me guessing that you are just resting the cd on the open drive. A little more of a description would be helpful.