Continuos loading circle when I turn on my Mac
I left my mac turnedoff for roughly a month. When I turned the computer on, the gray apple screen and the load bar appeared to start up the computer. Shortly into the loading process, the loading circle appeared (rotating circle seen to indicate loading in programs such as iMovie and Safari etc). The start up load bar disappeared and the load circle has now been rotating infinitely. I have tried pressing command+r and turning off the mac and turning it back on. I have plugged it into a power source. It won't let me past this gray screen. How do I get into my mac?
Thanks!
MacBook Pro, iOS 5, Bought in June of last year