I have a working wordpress installation at mydomain.com/wordpress/index.php (not a real domain). I want to migrate it to mydomain.com/index.php. How can I do that? I tried just unzipping a new copy of wordpress in the main domain space and copying over the config file, but I don't think I got it working, since it's not even giving me an error message--just a blank screen. Furthermore, going to mydomain.com/wp-admin/ seems to automatically redirect me to mydomain/wordpress/wp-admin.
You have to change Settings -> General -> WordPress Address (URL) and Site Address (URL) in the old installation.
WP looks in the DB at these settings and creates dynamic links, etc, from there, so you need to change it in the DB (either through these settings or by editing the db manually.)
Another option (and, actually, the better one) is to use a plugin which changes the links in the entire db - this will keep all of your links, images, etc. working properly.
I've used Velvet Blues Update URLS before many times, and it works perfectly. http://wordpress.org/extend/plugins/velvet-blues-update-urls/
Just remember - if making this change with the WP backend (through the settings page or with a plugin), you should do it on the old site. Then it should be immediately accessible at the new location. (Or you could just move your files to the new location, preserving your themes, media, etc.)
First, you might want to read prior answers to this question. Start with this list. Also, have you tried restoring the original
Second, what steps have you taken to update the database for the new install location? Did you export the database for the old location and import it to the new location's database?
Third, if you have imported the old data into the new database, did you take any steps to update the new database with the correct, current URLs? Wordpress databases contain several full path URL entries which you may need to update.