System Prefences->Start Up Disk (under "system" category), click the one you want.
We have a ibook in our computer lab running a Mac OS x 10.39 and I think one of my classmates decided he would mess around with it today. Now when you turn the ibook on a little box with a globe inside is blinking. Then it turns into a box with folder inside of it with a question mark. I think he might have put the ibook on network startup. How do I Change it back to booting from the HD.