MacBook Pro stuck on loading screen trying to boot from new SSD
Software causes
Possibly OS X might be booting expecting to have to work with the USB bus and getting confused by the fact it's now over SATA?
It in theory should handle that fine, but back in the day that did happen sometimes when making a clean install on a drive over USB and then moving it to SATA.
Never had the problem with Cloning an existing drive since then the OS was still set up like it was on the drive that you replaced, but clean installs sometimes would get set up in such at way that when it was no longer using USB to boot it would just hang.
So if that's it, running the install afresh should fix it up.
Just checking as well, is the SSD formatted as a GUID partition table, with a Mac OS X Extended (Journaled) filesystem?
Hardware Causes
If the above doesn't fit it, but the SSD works fine over the USB to SATA connector, then it might be a problem with the Hard Drive flex cable in your Mac, or possibly (though pretty unlikely, really) the SATA controller on the Logic Board.
Were you replacing the Hard Drive just because you felt like it was a upgrade, or were you having problems with it before. If you were previously having problems, the flex cable could very likely be your culprit.
Other than that, if you accidentally disturbed the Memory modules in your Mac, it is possible it might be hanging just because of that and removing and reinstalling them could get you up and running again. Best to se if you can start the computer from your removed hard drive or the Install CDs that came with the Mac first since if either of those work ok, then it is not the Memory, and you don't really want to handle those modules if you don't have to.
More Troubleshooting
Here is a bit more information you can update your post with that can help the folks here troubleshoot with you 

SSD model
How you formatted the SSD
Was the Hard Drive working fine before replacement
Was the computer powered down normally (Apple Menu &gt; Shut Down)
before the SSD was disconnected after you installed it
Can you start the computer by holding the Command and R keys