Sending e-mails to subscribers stopped working?!

Ok so I have a poetry site on wordpress (www.poetreecreations.co.uk) and a week ago I was able to send e-mails to all my subscribers at once and now I am unable to do this.

I’m getting this error message: ‘Message failed! Check your settings and check with your hosting provider
Could not instantiate mail function.’

I think the hosting is fine as the site is on-line and we’re still receiving notices and messages to the yahoo account that’s linked to the site.

I paid someone to build and setup the site and they are out of the country now.

I left someone in charge of it while I was away and they swear they’ve not changed any settings so……..

I’m just wondering if someone could run me through what I can check to get it working?

It’s linked to my yahoo e-mail address: poetreecreations@yahoo.com