Hi,
As you have replaced the H.D.D you have to install OS in it as well try below steps to re install OS in your MAC.
Insert the install DVD, restart or force restart if you cannot do that otherwise by pressing power button 6 seconds until mac shuts down, then press it normally to start.
Immediately after chime, press option/alt key; wait 3-4 seconds, you should see the boot options, among these the DVD and the internal disk, Select DVD and continue.
Go on booting from the DVD and, after selecting the operating language .
Follow on screen instructions to install OS in your MAC, Finish installation and restart your MAC.
Good Luck.