I’m getting the same issue since I updated to 1.5.7 theme version. I had to ftp in and rename the theme folder to be able to access my site and admin area again.
Edit – checked my PHP version and it is 5.4.24.
Edit 2 – Enabled debug and got the following error:
Parse error: syntax error, unexpected T_FUNCTION in **** /wp-content/themes/flat/functions.php on line 63
