e-mail function error – please help!

I didn’t change anything at my wordpress website but suddenly I get the following error message when using e.g. the contact form:

Warning: mail() [function.mail]: Could not execute mail delivery program ‘/usr/sbin/sendmail -t -i ‘ in /home/www/worldwidexchange/wp-content/themes/charitas-wpl/template-contact.php on line 36

line 36 of the template says:

36 mail($emailTo, $subject, $body, $headers);
37 $emailSent = true;

I did not change any of my settings. WordPress does not send any e-mails at all anymore (new user registration etc…)

Please help ;-/