Yep as others have said it's the White Screen of Death. This happened to my 770 three weeks ago. Rather unfortunate, but I did get a replacement under warranty from Nokia in under three weeks so it's not a total loss. The only unfortunate thing is I'm leery that the WSoD may show up on my replacement.

Anyway, if the 770 is still under warranty, (http://www.nokiausa.com/A4410053) send it off to Nokia and you'll get a replacement. Aside from replacing the LCD yourself, there's nothing you can do to repair it. Hope that helps.