Screen expanded 400%, can only see part. Too much scrolling!!

I was happily making posts to my new blog for several days. Then the screen suddenly expanded, so I can’t see all of the page at once. I can only see a quarter at a time. I can see the top left, or bottom right, etc. I spend all my time scrolling up-down or right-left to get to the functions I need which are all on the same page!
I have the problem on my laptop and on rented computers. I have a Dell running Windows XP. The consultant who set me up uses a Mac. This is so frustrating my blog is almost unusable.