Then your Recovery HD is also not working. Since that is a very old computer, you will need to start by installing OS X from the Software Restore DVDs that came with the computer or you can use the retail Snow Leopard DVD, if you have it. If you don't have either, then you need a Lion installer. You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax. It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.
You will have to get access to another Mac in order to create a bootable DVD or USB flash drive installer.