Assuming your laptop was working correctly (more or less - I assume there's a reason you replaced the hard drive), then it's likely that somewhere in the process of doing your repair that something got disconnected and/or failed to get reconnected.
So the first step is to eliminate what's changed; I'd unplug the new hard drive first and see if it works after that. If it does, swap in the original drive and verify that it operates correctly, or at least the same way it did with the original drive. Put the new drive back in and double-check that the system doesn't come up. If that's the case, then somehow you've got a bad drive and will need to get a replacement.
If taking out the new drive and plugging in the original doesn't make any difference then you're looking for a different problem. Retrace all of your steps and verify that anything you disconnected or removed has been put back together or reconnected correctly.
Go through those suggestions and let us know what you find, then we can figure out what to do next.