Drupal 7 "undefined function" fatal error; from FTP error
If you have a backup this is the time to use it.

If not you should:


Put your Site in maintenance mode from command line: https://drupal.org/node/590468 or by using Drush: drush vset maintenance_mode 1
Replace the deleted module with exactly the same version in exactly the same place.
Run update.php either from drush: drush updb or command line: https://drupal.org/upgrade/running-update-php
All should be fine. 


Please report back if this works or if another solution is found