[SOLVED] upgrade/activation of 3.5.3 caused blog to go blank/white, can't login to wp

OK, guess I am in a pickle. I needed to update from WP 2.8.2 (at least that is what the wp-includs/versions indicates) but first decided to update Atahualpa from 3.4 since it should be easy to go back if I mess that up...

Except as soon as I activated 3.5.3 my site went to a blank, white screen. The blog address (intranet) shows the same, wp-admin shows the same, etc.

How do I manually switch back to 3.4 and start the process of figuring out why 3.5.3 caused this? All I did was unzip 3.5.3, copy a header image into the directory suggested in the readme, FTP it up to my server, log into wp-admin and clicked on activate Atahualpa 3.5.3.

Thanks.