Can't access site due to redirect loop??

I have just installed and set up various wpmudev plugins on my first 'proper' wordpress site (I'm not a techie - learning it all as I go).
I'm not sure what I've done wrong - I set up a few web pages and a menu to display them and suddenly I can't go to the site ( as it says something about a redirect loop.

Any ideas what I've done wrong and how to fix it (can't obviously get to dashboard either to try and fix it)?