Mail Function Not Working

-- url of the page with the problem -- : morenadancewear.com.au
-- HikaShop version -- : 2.6.0
-- Joomla version -- : 3.4.8
-- PHP version -- : 5.5
-- Error-message(debug-mod must be tuned on) -- : Notice
Could not instantiate mail function.

Could not instantiate mail function.

Hi,
It seems that Hikashop mail engine is not able to send emails to the client (and also to the nominated admin email) once the checkout process is complete. I am getting a notice stating that "Could not instantiate mail function" (see picture attached). This was working perfectly well until yesterday. Hikashop is configured to use the PhP Client (see picture attached), I am not really sure if the problem starting to occur because of something been changed on my server (by the server administrator) or because of something else. I contacted my server administrator and they told me to contact you first to seek assistance and also asked me to provide any information that can be useful for them to check if the problem was caused by the server. This function has been working very well for as long as I have been using Hikashop, it only stopped working today.
Any help will much appreciated.
Regards.