Thanks for your excellent answer here.
When I say suddenly I mean about one week.
I think my main concern is not so much that my domain has dropped but that the pages which I want to be successful have fallen way behind less important pages on the website such as the personal training packages being above both the personal training page and the homepage.
Previously the Personal Training page was the highest rank followed by homepage and these have completely fallen off.
It is worth pointing out that my homepage has a domain authority of 32 and my personal training page 25. The domain authority of the pages ranking best and which are less relevant is less than 16.
Thanks again for your help.
Daniel