You may have been affected by this issue (present in the 1.4.0 release and manifesting itself on servers still running PHP 5.3 or older).
If this is the case and you are comfortable with FTP, download the 1.4.1 update and replace your wp-content/themes/nirvana/admin/sanitize.php
file with the one from the zip.
Afterwards you should be able to access your dashboard and install the complete theme update.