Fatal error updating 2.5.17

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I updating to 2.5.17 and received..

Fatal error: Class ‘Ai1ec_Exception’ not found in /home/******/******.com/wordpress/wp-content/plugins/all-in-one-event-calendar/all-in-one-event-calendar.php on line 27

killing my site. I can’t do anything to work around it or restore a backup.

This is the text around line 27 from the .php file

if ( ! function_exists( ‘ai1ec_initiate_constants’ ) ) {
throw new Ai1ec_Exception(
‘No constant file was found.’

Help!