There's a known workaround for this issue. Close autocad and reopen it.
I have fairly regular random screwiness in autocad sessions. The more drawings that have been opened during the session, the greater the chance. The most common problem I get is that the dialog boxes stop working. Having objects that are on, thawed, unlocked, and completely unselectable isn't as common but it's not overly rare either. FWIW, I've been seeing this behavior since 2006 although it is far more common with 2008. I have never figured out a common denominator but like I said, there seems to be a greater chance when several files have been opened.