Go to Start\ right click computer\ click manage\ in the window that appears, choose disk management.
In the window on the lower right, scroll through the list and see if you see the CDrom, if it is there, what drive letter is it using, if any. If it is there but no drive letter, right click on it and choose Change Drive Letter and Paths, choose an open drive letter.
If it is not there, check out your device manager on the left above disk management, do you see anything with yellow exclamation marks?