Hi,
It's empty that means it has nothing to boot. You need a bootable disk or disc and install the Operating system on your new HDD. The quick and easy way is to order a new recovery set from HP. Pleace call HP technical support: 800-474-6836. If you live outside the US/Canada Region please click the link below to get the support number for your region.
http://www.hp.com/cgi-bin/hpsupport/index.pl
Regards.