iMac won't start up.. Stuck on blue screen! Help, please.

My iMac won't start up. After the power went out and I attempted to restart it, the screen would first display the white apple page, then change to blue with the spinning circle, and then stay that way. This is not the first time the power had gone out, but the first time the computer did not start up. I long-pressed the power button to shut down the computer and waited a minute before attempting to restart it. It stayed frozen on the blue screen once again. I'm not sure why to do. I'm on my iPhone right now an have been reading random posts here and there about holding down certain keys while attempting start up or checking plug outlets and power source. Please help!!! All my work is on my computer and final exams are in less than a week. Actually [most] of my life is on my iMac. What can I do???