As I understand it, Windows 7 did start running normally, but has since failed to load. So the upgrade has worked, but the quality of that upgrade is suspect. The problem is probably a faulty driver, so your first port of call would be to disconnect all devices (printers etc) and then trying to load in Safe Mode. If this works, then try running Windows normally, adding your devices one at a time and then rebooting, until you find which device is causing the problem.
If Safe Mode does not start, you will have to try to re-install Windows 7 over the top.