waf/bootsrap.php – failed open stream

I have had two sites go down in the past 12 hours. Both came up with the following error:

Warning: require_once(waf/bootstrap.php): failed to open stream: No such file or directory in /home/content/p3nexwpnas05_data01/50/2910950/html/wp-content/plugins/wordfence/wordfence.php on life 54

Fatal error:require_once(): Failed opening required 'waf/bootstrap.php' (include_path='.:/usr/local/php5/lib/php') in /home/content/p3nexwpnas05_data01/50/2910950/html/wp-content/plugins/wordfence/wordfence.php on life 54

Had to go in via FTP and disable WordFence to gain entry into site again. Any clues as to what is happening here? Both sites have been running WordFence for a while and get updated regularly.