Try holding down the trackpad button on startup and see if that ejects the disc. Holding down the mouse button on older systems used to eject floppies on startup.
If not, the simplest solution would be for you to get a paper clip and push it into the small opening right next to your dvd drive until your disc ejects. It's difficult to see unless you look for it, but it's there and it was expressly created for situations such as this.
For details, refer to your manual - it's all in there.
Hope that helps, and good luck!