[support] Suddenly, my 404 page is stripped of all its blocks, menu, etc

I upgraded to 5.10 yesterday and the only discernable thing I've noticed is that my custom 404 (mysite/404) page has all its blocks stripped out...why would that have happened? I don't use menu module - I use a block for a horizontal two-level menu and that's gone. The header block with search/login, etc has gone and the footer block with contact/faq/copyright, etc has also gone. anyone want to help me solve this mystery? :-( Neil