Might want to try booting into “Safe Mode” by pressing the F8 key immediately after the computer’s boot logo has appeared. You can then select from the various options that appear in the menu that is displayed. Sometimes just booting into Safe Mode and then trying a regular reboot again will prove successful in displaying your regular desktop. If that method is unsuccessful then continue troubleshooting via Safe Mode (for example, disabling or uninstalling any new software installs or devices that maybe a source of your problems, run tests on your hard drives etc).