Can't access my site - Server Error 500


I was on the Dashboard of when I noticed the summary of number of Posts etc. I saw that it said 1 Comment so I clicked on that and ever since I can't access the back-end of the site. Before clicking on it, I could see lower down on the page that the comment appeared to be an introductory comment 'From Mr Wordpress'. The home page eventually loads after an age but the menu formatting looks odd. When I try back-end pages like /wp-admin I just get 'Server Error 500. We have two other sites hosted on the same server, and they are not experiencing the same problem.

Thanks for your help.