500 Internal Server Error-Cannot Log in to site

I am unable to log into my site to remove a plugin that may be causing the problem. My hosting company has done all they can. The site keeps breaking and creating this error.
site: http://www.tutorprofessionaldevelopment.com
I am using a plugin Ackivia so that my visitors can sign up. I think that is one with the buggy issues because I did not have a problem before that.

Thank you,
La Vergne