i had the same problem today, i upgraded from wordpress 2.6.5 to wordpress 2.7.1
and i was surprised to see my admin panel gone and i cannot see the theme editor. it was useless.
for 6 hours, i searched for answers how to restore and how to downgrade, and by trial and error, finally i got my old wordpress 2.6.5 . Yehey!!! now, i have a lesson not to upgrade right away when told to do so.
this is what i did,
1. download your preferred old wordpress from here.
download, unzip and extract it. remember where you save the extracted file.
2.open filezilla, go to public_html, delete the wp-admin (of wordpress 2.7.1)
3.in filezilla local site, look for the wordpress file that you downloaded in step 1, right-click wp-admin, and upload.
4. close all tabs and open your own http://yourwebsite.com/wp-admin
(replace yourwebsite.com with your own website name)
and voila!!! if it worked for me, i hope it will also work for you...