If you are unable to login to your drupal 8 site then reset the password for your user using the following steps

  1. In your Drupal root folder execute the following command
$ core/scripts/password-hash.sh  "your_password"

You will get a hash value as shown below

password: your_password   hash: $S$E5o2XTgRd4E3QyEdVyQlNrc3WCvBr44T/UGrAYXSNaU6ZH5ikJAu
  1. Open phpmyadmin and select your database and find " users_field_data" table. Update the hash value with the new value generated in step 1

  2. Truncate/Empty "cache_entity" and "flood" tables from database

Now you will be able to login to your site with the new password. Hope this helps :)