Cannot edit pages or add new pages

Hi, I'm running into an odd problem. I am using the Atahualpa theme and have a static page set as my homepage and the blog posts page is set to "/news". After messing around with the site a bit, I suddenly could no longer edit pages...when I click "edit" I get a blank screen in the admin area. I also can't work any of the setting on the right-hand side - edit publishing status, set date, etc. Attempting to add a new page also results in a blank center content area in the admin section.

I tried switching themes and then switching back to Atahualpa, and now my homepage content is completely lost.

In the process of all this, I was getting frequent internal server error 500 warnings.

Can't figure out what in the world is going on...could this be an incompatibility issue between either Atahualpa, Wordpress 2.9 & my installed plugins?

Thanks for any tips.