Did you get any errors during the upgrade? There are a few other posts on this. Look through a few days and you may find the answer.
You can still get earlier versions of WP, by the way, so if you decide to go back using a backup you should have made before the upgrade and restoring your database you should be back in business. Just click the Download tab at the top of any page then select the archive folder from the left and every release of WP is there in ZIP or TAR formats.
BTW, a casual search on similar problems – which you should have tried before posting a question – reveals this answer from moderator Micheal:
“Here’s some things to try:
1. logout or logout with https://www.yourdomain.com/wp-login.php?action=logout
2. disable all plugins–if necessary read How to deactivate all plugins when not able to access the administrative menus?
3. clear cookies
4. exit browser
5. make sure local firewall is not causing problem
6. login with https://www.yourdomain.com/wp-login.php
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.”
