I found the following rough sketch of what could have happened written by an alleged 777 pilot on another forum.
He claims that with some equipment failing, possible smoke in cabin they were in alternate navigation meaning they would have to key in all coordinates (no navigation database) in raw numbers into CDUs, they probably made some entry errors and ended up flying to 'nowhere' while finally overcome by fumes.
I personally don't like this theory on multiple grounds. Would a professional pilot in dire emergency with malfunctioning equipment around him attempt to key in potentially 8 numbers into an FMC? The weather was VFR, they could have made a 180 turn using either HDG select or just flying manually and since they were still only 40 mins into the flight lights of Kuala Lumpur would probably be visible. Why would someone attempt to use any kind of RNAV, wasting time heads down keying in some numbers into a suspect equipment while being still relatively close to land/airports.