iMac won't start up, stuck on flashing question mark folder?

I am not sure any exact specs or information on the computer and can't check on the computer itself because it won't start up.. it's the 23.5? Inch and I believe I got the computer late 2009 or early 2010 and recently upgraded to El capitan. Everything was working fine until I left a game running and my computer went to sleep and hours later when I came back to it, it was frozen. I had to hold down the power button to shut it off and then when I started it up again it would go to the gray screen with the apple logo and show the progress bar, very very slowly it increases then once it gets half way it just stops and that's it... it won't ever start up since then... I tried everything I saw holding command and r, and shift and holding D and option control p + r... or whatever (I know nothing about computers) and nothing would come up. It only kept going to the logo. Idk if it's because I have a wireless keyboard and it can't detect my key presses in time... but now the apple logo stopped coming up, now it just sits on the gray screen for a long time and then starts flashing a folder with a question mark on it.. idk what to do anymore. Someone please help. I can't get it to boot up whatsoever! !!!