I'll list the full suite of stuff to try in case there's something you haven't done yet (do these in order):
Log out if you can.
Clear your browser cache and cookies (or go incognito/private in your browser)
Set the following System Settings in the modx_system_settings table to these values:
manager_theme: default
compress_css: 0
compress_js: 0
In some upgrade situations, there are three System Settings that should be empty or missing, if you find them, empty the value field:
session_name
session_cookie_path
session_cookie_domain
In the modx_site_plugins table, set the disabled field for all plugins to 1.
Delete all files in the core/cache directory.
Run Setup again in upgrade mode.
If you went incognito, stay that way to test the site. If it works and *not* being incognito breaks it again, log out, clear your browser cache and cookies, and delete all files in the core/cache directory again.
If the above works, re-enable the plugins in the Manager one by one to see if anything breaks.
If none of this works, the you may have a missing or corrupted file in the install. This is common if you transferred them individually with FTP. See this for another method:
http://bobsguides.com/modx-upgrade-faq.html
If your problem is downloading packages after an upgrade to 2.3, the problem is likely mod_security at the host.
