Wont boot past the gray screen with the apple logo

How can I fix this myself it was booting up to the os utilities page I tried everthing on the window, then I restarted it and hit the option buton it went to the to disk os based system and recovery I selected the recovery disk and tried it that way. Now im trying to go back and change it back and it wont go. I even tried to restart it in safe mode and couldnt get it to work. I need some tips and advice on how to correct this problem. How do you go to the system info , I know you do it as you restart it a certain combination of keys