Did you just install a new hard drive to replace your old one, or in addition to your old one? The path we go down depends on that. It would also be nice to know what kind of computer you have.
Another important question: if you remove the new drive and have only the old one in the machine, will it boot then?
If you installed the new hard drive instead of the old one, a hard drive comes blank and you have to transfer the system from the old drive by "cloning" it or, better, re-install the OS and everything else on the new drive.
If this is an additional drive, the problem may be as simple as your system trying to boot from the new drive, which is blank, instead of from the old one.
Third possibility: One of the drives is not installed properly. If this is not a notebook machine, this would be a good time to check / reseat power and data cables and, if you are familiar with BIOS, boot into the BIOS and see how it detects the drives.