Post titles disappeared on blog page

Hi, For some particular reason the post titles on my blog page and atop my individual posts seemed to have disappeared. Not so long ago I installed a SEO plugin and at first I thought this was the reason why the post titles had disappeared but when I deactivate the plugin the titles won't reappear. Furthermore, after I installed the plugin the titles were still visible on the blog page and atop the posts. I haven't made any adjustments since I launched my website a month ago nor have I used the SEO plugin as of yet. Could you think of any reason why the post titles could have disappeared and how I can fix this? http://www.pennenspinsel.nl/category/spinsels/ Thanks in advance! With kind regards, Tabitha