#35 Error when open Calendar

[Anonymous]

Hi folks, it was running smoothly until today. Suddenly it shows the following errors when I open the ExCalendar this morning:

Warning: main() [function.include]: Failed opening '' for inclusion (include_path='.:/usr/local/lib/php') in /home/hklaorg/www/www/components/com_extcalendar/lib/mail.inc.php on line 3
PaTcHeD By DiO

Any advise on how to work out? Many thanks.
Kevin