screen size reset - my imac screen is too large

My 12 month old banged the keyboard in my imac for about 20 seconds. The result: it looks like the computer was reset for a screen size LARGER than the imac screen size.

I now need to use my mouse to see the full screen, e.g., to see the time display in the upper right hand corner, I need to take the mouse there so that the screen moves to the upper RH corner. To see the dashboard (which I keep in the bottom), I need to take the mouse and take it down in the screen, so that it shifts down.

I made sure the display resolution is 1600x1000, but it had no effect on my problem.

What should I do.