wp-content/upgrade created automatically

Hello

I have been editing a theme using a child theme. I was playing around with the background image – nothing complicated – then the site suddenly appeared blank.

I looked at the file directory and I noticed that the ‘themes’ folder had been moved from:
..wp-content/
to
..wp-content/upgrade/

Does this sounds like a security breach or something else?

I’ve tried re-installing WordPress 3.5.1 but it had no effect.

Any suggestions on how to fix this or what might have gone wrong would be much appreciated.

Thanks in advance.