I updated to new version of Matalb R2018b (184.108.40.2069112), and when click on the "Help" ikon I get a window with "Page not found". (I could access offline documentation help pages with my previous version.)
As far as I could understand a similar question has been posted on MATLAB Answer. You can see the thread here and try to follow the solution give there. The error message displayed on the blank help page in that case is slightly different but I believe the solution should help in your case too.