SSL Certificate error - can't view website

One of my clients recently changed their website to wordpress and the domain host also changed. I am now getting a certificate error when I try to visit the site and Safari tells me the site is unavailable. I know the site is working because I can view it on safari on my iphone.

I have had the domain check their set up and it is all correct.

No one else is having this issue. It only seems to be my macbook that won't allow the site.

I've tried clearing all my caches including system caches.

Can someone please help?