My text is now without line breaks

i have been blogging for over a year with no problems, except for tonight. I toggle between my visual and HTML view in my post editor… but am no means an HTML coding master. I can only make very simple changes.
In my visual view I just discovered the “fullscreen mode” button. I clicked that and pasted some text in. I also clicked the “remove formatting” button in hopes it would do something that I was proven wrong in by the experiment, and tried different formats from the drop down menu. I kept the format on “paragraph” however, which is what has always been chosen for all my previous posts.
But, now my text goes out of my outline and looks like this

i tried troubleshooting on the wordpress site and sifted through a lot of possible problems. I just can’t seem to find the answer.
What have I done?
I’m not sure what has changed, my test has always automatically fit within the fixed width, but doesn’t anymore.

I also have already tried writing text while not in the full screen mode and in the HTML and visual text editor window. Still, it does not automatically format to the right length anymore as it used to. ??

any suggestions would help!
thanks so much!