This is a very common issue which happens when WordPress was not successfully updated. If this happens, then the admin is not able to log into wordpress dashboard as it redirects to this url Problem Solved: upgrade.php?_wp_http_referer=%2Fwp-admin%2F .

Below are simple steps to fix this issue in 2 minutes.

1. Open File manager through hosting account or using FTP.
2. Go to the wordpress installation directory and under wp-content folder, rename plugins folder to plugins123.
3. Then go to your website url and write /wp-admin after the url.

You will see the dialog as below:

Click on UPDATE WordPress Database and the problem will be solved.

Cheers and comment for any issues.