Macbook Pro 13-inch stuck at Gray screen with apple logo and wheel I have a 13 inch Macbook Pro, early 2011 with its original OS.
I came to a point where its battery needs to be replaced, and I can't use it without using its charger/adapter. One time, I  accidentally  pulled out the charger and it turned off. It was not my first time so I just didn't worry. But when I plugged it back and turned on the laptop, it just got stuck at the gray screen with the Apple logo, the wheel, and a loading bar. At first I thought I should just wait, but later I realized it's still stuck for almost an hour! 

I tried to Google and I read that something is corrupt. 

I don't really have too important files, so it's okay if I would need to reinstall the OS. Or do I need to change the laptop's hard drive?
Well, seeing as the laptop is at least attempting to boot from the hard drive, I'd say that your hard drive is fine. 

My recommendation is that you reinstall Mac OS X. An early 2011 MacBook Pro would've come with Mac OS X 10.6 Snow Leopard, so you should have had some DVDs come with the MacBook Pro when you bought it. Use those to re-install Mac OS X 10.6.