Paragraphs disappear after saving

Hi there,
When I writing in visual and then save, all of my paragraphs disappear. This seems to happen most obviously when I switch between visual and text. I only use html to embed pictures and videos. I have been doing this for ages with this account, but just recently tried embedding an image from flickr rather than downloading it and all of a sudden this problem appeared.

I do not know how to use code. This blog is used for a course and I need to post my assignments!