Please see http://twoweeksintuscany.com.

Advice on repair would be appreciated.

What version of Atahualpa, WP and PHP?

take a deep breath - this can be fixed! did you add a plugin? or a new widget area?

If you added a new plugin, go delete it from teh plugins folder and try again, otherwise:

1) use an FTP program to make a copy of your Atahualpa folder from the themes folder (always make a backup - in case you've added any images etc)
2) delete the atahualpa folder from the themes folder - make sure the themes folder still has the default theme
3) see if you can access your site

next it depends on what you did to your site before this error.

What did you do before this happened?

I think that all I did was upgrade but I am able to log into the Dashboard so I can easily follow your advice.

Question, should I delete plugins or merely deactivate?

Thankyou for your help.

Deactiving plugins did not work. Deleting, likewise.

I think this may have been triggered by an failed auto upgrade of either WP or the theme. I'm thinking it's the theme. Do the error msgs support that idea?

yes, you should be able to swap to the default theme. Then

1) use an FTP program to make a copy of your Atahualpa folder from the themes folder (always make a backup - in case you've added any images etc)
2) delete the atahualpa folder from the themes folder
3) download a fresh copy of the theme.
4) unzip it and upload it to your host
5) activate it and see if it works