Had same problems with my site. First thing you should do is to go into your ow_includes/config.php and turn on OW_DEBUG_MODE:
define('OW_DEBUG_MODE', true);
That should at least give you some idea as to what the underlying problem is. What I did was go into the oxwall database using phpmyadmin via my cpanel and disable all the active plugins by setting isActive to 0 in the ow_plugins table. Then do <site>/ow_updates and see if you can at least get into the admin area. If not, post what you see after enabling debug mode.