either I get a 500 Internal Server Error or I can't access the dashboard on my wordpress websit

Hi, My site: http://distinction-review.com seems to be having a serious issue. and Hoping someone can help.

I either get a 500 internal server error or if site does load I can see pages but can't access back end. i.e. dashboard.

I am in serious need of help.
Thanks so very much.

Mark Moore