Did you deactivate plugins before you upgraded and switch to default theme.
If not
Try this steps WordPress
1. deleting/renaming your .htaccess. it’s possible that a .htaccess rule could be the source of the problem.
2. deactivating all plugins to see if this resolves the problem? If this works, re-activate the plugins one by one until you find the problematic plugin(s).
Since you can not log in admin panel using ftp or server rename plugins folder to plugins-hold and create a blank plugins folder and try and see if you can log in. If you can log in then using ftp delete the blank plugins folder and rename the original back to plugins
3. switching to the default theme to rule out any theme-specific problems? If you don’t have access to your admin panel, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the Default theme (Twenty Ten theme) to activate and hopefully rule-out a theme-specific issue
4. resetting the plugins folder by FTP or phpMyAdmin? Sometimes, an apparently inactive plugin can still cause problems.
5. re-uploading the wp-admin and wp-includes folders from a fresh download of WordPress?
Thankyou.. your advice helped me restore my site.
