No idea what’s wrong…

I mistook a friend’s advice on upgrading my site, thinking he said “reinstall” wordpress rather than “get a new copy of index.php,” which is what he actually said.
So at the same time I tried to change stylesheets and I “reinstalled” WordPress, and now it’s jacked. However, when I pointed my browser to “wp-install,” it said “it appears you already have WordPress installed, delete your database and start again.”
Well, obviously, I don’t want to delete my database, so I have to fix what I broke some other way.
Here’s my site.
The error message I get at the top of the page is:
Database error: [Unknown column ‘post_date_gmt’ in ‘where clause’]
SELECT DISTINCT * FROM wp_posts WHERE 1=1 AND post_date_gmt <= ‘2004-08-29 00:56:59’ AND (post_status = “publish”) GROUP BY wp_posts.ID ORDER BY post_date DESC LIMIT 20
And where my posts should be it says “Sorry, no posts match your criteria.”
Any idea how to fix this?
thanks