My sidebar just moved! What happened?

I was editing a post from my dashboard and suddenly my sidebar was gone! It has moved down to the footer area. There’s a screen option button but it only allows 1 and 2 columns. the “1” column gives a full screen view and the “2” compresses the screen with space on the right for what was there before and now appears at the bottom, i.e., Publish, Post Tags, Categories, etc.

What did I do?
Carlos