Ok try this. When you're at the black screen press Ctrl+Alt+Delete (all at the same time), and when another screen comes up click on Task Manager. When Task Manager opens click on File -> Run (or something like that, just look for the one that contains Run in it), and then type in explorer.exe and press enter. Everything should load up now. This has happened to me before and that's usually what I do and it works for me just fine.