How do you fix that thing where your browser disappears from view?

I pressed some ons, and I know I've done this before, but for the life of me I can't remember how to fix it. My browser disappeared, like it left the screen so that I can see more of the page. It reappears when I put the cursor at the top of the page, but I like having my browser there 24/7 so that I know there isn't something wrong with my computer. How do I get it back to normal, where it's showing all the time?