Newbie n800 owner of 2 days and this has hapened twice.
Had autolock enabled, key/screen locked, when I came back could not unlock as center of d pad would not work. My cure/workaround was to plug into ac to charge. d pad then responded. I should have had enough charge in the battery, but it's possible this is a factor. For now I have disabled autolocking and will see how it goes.