Not sure if this is the answer but I did that once and I found out that I had hit the percentage box at the bottom right-hand corner of the screen. Try this and you could be fine - note, I did say could !!!!
Hold down ctrl and scroll up or down with your middle button on your mouse...Or, on a touchpad, again hold down ctrl and use your scroll up or down to alter it to how you want it
If its on here the go to the top right corner, above your login name and there is *A* click on this to make it bigger. On a tablet it is three = like this. On an ordinary laptop it is under the address bar in a drop down window.. Hope that works for you.