I was having a similar issue – the fusion builder was not showing, the button for the default editor was showing but not actually allowing me access to the editor. Additionally, hitting the screen options button at the top of the page wasn’t bringing out the dropdown.
I deactivated all plugins with the exception of fusion builder and fusion core and all was fine again. I then reactivated the other plugins one at a time and eventually found the conflict – which, in my case, was Yop Poll.
All working fine now. Obvious a conflict has only come about following the new release of fusion builder.