My hp zd8000 is not rebooting and been told i need

Before you start thinking about your motherboard, let's rule out your harddrive.

Go online and Google "emergency boot cd" and choose a product that works for you. Many have "demos" you can use with limited capability, but that's fine. You will need to take the .iso file and burn a CD with it. This will create a bootable CD.

You will need to setup the computer to boot from a CD by going into BIOS and setting the boot sequence to CD first. If you don't know how to do that, here's a good article...
http://www.cites.illinois.edu/security/antivirus/bios.html

Using your newly created CD, put it into CD drive and start boot up. If your computer boots normally, you have a harddrive tht has failed. If not, you may have a motherboard issue, or possibly a different hardware issue that may require professional assistance.