Upgrade to 3.8.4 gives White screen
 

I just upgraded 3.8.2 to 3.8.4 - everything seemed to go well* although I did notice that in the admincp that I seemed to be missing a few links on the menu bar to some of my plugins etc but they're in the options list so I figured no bother. I then reactivated my forum and guess what - nothing but white screens. What did I do wrong? How do I even begin to sort this out???