VAN HIRE on "I cannot get into wordpress admin - Server Error 500"

Hi

I cannot get into word press admin - I am getting the following message when i login - SERVER ERROR 500.

The host are saying they have done nothing 0 but will charge me $48.00 to roll back - I have done nothing to change the site.

Can anyone help me with a fix please or some advise.

Here is the link:-

http://vanhirewoking.com/wp-admin/