You might have just changed the "zoom level" - I've done it several times by mistake.
Look in the bottom right hand corner whilst using internet explorer (like right now). Just above where the clock is there's a little magnifying glass and a % zoom, usually this should say 100%. To change it, click on the dropdown arrow next to the number and select 100%....
It might be something completely different though. I'm assuming you "pressed some buttons" on the keyboard rather than the monitor?
EDIT - Simonta - you beat me to it!!