Check the server log as Andy Tough suggests. Also check the MODX error log at core/cache/logs/error.log.
Be sure to delete all files in the core/cache directory and try visiting the site with your browser in private or incognito mode to rule out issues with the browser cache and cookies.
If none of that helps, it may be a plugin or widget that doesn't play nice with the new version. You can bypass the widgets by going to this URL:
If that works, you can remove all the widgets from the default dashboard, clear the cache files again, and add them back one at a time until you find the culprit.
If not, you can disable the plugins in the modx_site_plugins table. Clear the cache files again before trying to log in.