installing PHPmailer in CI2 third_party folder

hello everyone

I coded my website to send emails using the default mail() function.It worked even with html format emails until a day ago that i recieved the error message:
Unable to send email using PHP mail(). Your server might not be configured to send mail using this method. What can possibly be the problem? I need your help please. I'm also considering using PHPmailer or any other third party mailer yet i don't know how to install it. Thanks in advance for your help.