hey elvira,
no worries, you can take your time to search the code and update later without rush
I already found the issue and fixed it at your site. turned out - for whatever reason - some of your blog posts were not associated with a user and therefore ran into this error as the system couldn't show the blog post author. I set all missing entries to your administrator account (of course you can assign them to anyone else if you like) and now the posts show up fine again.
hope that helps and have a nice day