Make sure the CD burner is plugged in.
Make sure your CD is blank and valid.
Always check your CDs on another computer before leaving the lab (if you'll be using the CD on a MAC, check it on another MAC; if you'll be using it on Windows, check your files on a Windows machine).
If your CD doesn't work and you get a message about the disc being wrong or missing, push the CD back in (sometimes the program just doesn't read it). If it still doesn't work, your CD could be bad. Try another CD because blank CDs often have problems.