MacBook unable to boot.

I am trying to reinstall mountain lion onto my MacBook Pro (2008) from a USB as I reset it as was trying to download OS X El Capitan. However when I try to boot up my computer, the question mark with folder appears. I have tried command R and the alt method but nothing is happening. Is there a way I can fix it?

I trust you've backed up all your data...?

The ? means that the operating system can't be found. Are you pressing cmd-R immediately once you've powered on?