Writing a post – the text continues writing of the screen, and another problem!

Hi there,
a relatively pair of problems, which were not an issue until recently.

Problem 1
When writing a post, the text carries on writing endlessly to the right without dropping down onto the next line. When I publish the post the text just disappears off the right hand side of the screen.

Problem 2
When writing/editing a post on my WordPress dashboard, if I press enter the cursor jumps two lines instead of one. When I publish the post it also appears like that on the website.

I haven’t (knowingly) changed anything in the style.css that would cause something like this to happen, but if anyone has any suggestions I would gladly welcome them!