It looks as though the proc was in a non-signalable state.
This often happens when e.g. you have a lingering stale NFS mount because the server disappeared, or when a disk drive passed away and needs replacement or because a filesystem became corrupt.
Usually the hanging procs terminate as soon as the resource they have been waiting for becomes available again.
Of course, as someone mentioned, you can always reboot to flush your proc table. But I guess this isn't really an option?