macbook wont boot after recent update.

So my App store gave me a notification that there is an update and as per the Apple guy from the store told me "Always update your OSX". So I followed him, I'm a newbie with Mac so I wen't along with it. After the update, I was doing what I usually do, everything is smooth. Turned it off before I sleep. The next day, I turned it on and it stayed at the Gray screen where it has the Apple logo.. there would be a wheel and after sometime it would disappear.

So I tried doing a long press on the power button, hoping it works like a windows and restart, it did.. but i was the same thing.

I wanted to bring it back to the store but I couldn't leave, last option is hope that someone would read this and answer.

By the way, my Mac Pro is only 7 days old, should I exchange it for something new? Do you guys think it's one of the bad batch? HELP, PLEASE..