Permission Settings

Basically what happened was I fiddled with the permission settings of the index files in my FTP client located in the worpress-admin folder. I was trying to customize a header in a theme I downloaded, but wound up screwing up my website. Now the site looks like it’s half-loaded and the theme I downloaded won’t load to my site. Some things in the dashboard will load, but the themes won’t. What can I do to get my site back to normal?