iMac Yosemite. After booting up today I kept getting the pop-ups asking for password. Was going thru the steps and the entire system proze up with the spinning rainbow ball. Only can run 1 program that is Firefox. Alos can't print anything.

My system is iMac, Yosemite & I am a fairly recent newbie to iMac. I was trying to solve the problem of the multiple popups asking for passwords. While accessing Keychain Access my entire system froze up. Now all I can do is watch the rainbow spinning ball. With each program I try to run the ball keeps spinning. I have forced close on every program but the ball keeps spinning on all programs except I can run Firefox. And I can't print.

What can I do?