Upgrading–no login

upload the contents of the /wp-admin folder again

This can also be a plugin issue.

Delete any cookies, refresh cache, and try again.

Here’s some things to try:
1. logout or logout with https://www.yourdomain.com/wp-login.php?loggedout=true
2. clear cookies
3. exit browser
4. make sure local firewall is not causing problem
5. login with https://www.yourdomain.com/wp-login.php
6. disable all plugins–if necessary read How to deactivate all plugins when not able to access the administrative menus?

If WordPress is installed in something other than the root folder please take that into account for the above url examples

Please let us know what does solve your problem.