If using a paperclip to activate the manual release doesn't work, the drive is probably shot...the gear that opens and closes the drive is broken, rendering the drive unusable. Since your computer is still under warrenty, e-mail, IM, or call HP and let them know what happened. They'll probably have you ship the drive to them, then either repair or replace it and ship it back (they'll pay for shipping and handling).
Hope this helps,
I have an HP A810N and there is no CD in the DVD burner, and the tray will not open, I tried opening with the manual pin hole, but that did not open it.
Have any other suggestions.