customize a theme

Hi
I was just wondering if someone could help. I downloaded wordpress to my laptop and I have logged into my site to customize it and start adding posts etc, When I click on the customize a theme, all I get is the side bar, and the loading symbol, but the theme never loads and it wont let me edit anything, even the title when I change it and click save nothing happens, still just the loading bar displayed??? this is probably something very easy, but I am completely new to all of this so would appreciate any help.