Email script not working

part of the code in my script that is meant to send emails are not going, from the registration script and different script .they all used to work fine. please is there any changes that might be affecting them.

part of my registration script is below please.

       //generate random code
       $code = rand(11111111,99999999);

       //send activation email
       $to = $email;
       $subject = "Activate your account";
       $headers = "From:";
       $body = " Hello $fullname,\
Username $username,\
 Password $password ,\
You registered and need to activate your account, n\
Please keep this email safe as it contains your password . Click the link below or paste it into the URL bar of your browser\

       if (!mail($to,$subject,$body,$headers))
           echo "We couldn't sign you up at this time. Please try again later.";