Few things to try. Right-click the drive in file explorer and click eject. Reboot and try to eject during post. If those don't work, use a paper clip or similar in the manual eject hole in the cd/dvd-rom drive.
You should post your exact model computer. Newer models' dvd drives do not have an eject opening. The cd might be in the drive incorrectly such as crooked and that is what prevents it from ejecting. Check your service manual at Dell Support to see if you can remove the drive if nothing else works.