Please help: Featured Images not showing up in posts or pages suddenly

I’m experiencing a weird problem. Basically I’ve just installed the Magazine Basic them on my site and after spending a day editing the CSS suddenly the featured images are not showing up on posts and pages. Try as I may I cant find a solution for this.

I’ve tried a few things, such as downgrading to an earlier version, disabling plugins (even renaming the plugin folder temporarily, resizing images and a few other things but cant for the life of me get this sorted out.

Whats worse is that even if I install another theme now, the same thing is happening. The only theme that still displays the featured images is Easel, which is the theme I was using previously.

I’m at my wits end. Can anybody help?