The issue is similar to Magento 1.x which we posted last year. It happens when you migrate a database from one instance to another. In this case frontend of the website loads without issues but when you try to open Magento 2 Admin it will show the error 404 Not Found.
Magento 2 Admin Not Found After Database Migration
How To Solve Magento 2 Admin Not Found Issue
In order to solve it, you need to execute SQL query at your database. You can use a tool like PHPMyAdmin or Navicat. If you have direct access to your database you can execute it as well or in another case, you can make .sql file and import it into your Magento 2 database using a command line. (mysql -u user -p magento2_db < file.sql)
Following SQL code will reset default values and you will be able to access your Magento 2 admin panel.
In case if you still experiencing the issue, try to refresh Magento cache using the command line. For this as Magento file owner launch command:
bin/magento cache:flush or php bin/magento cache:flush
After cache will be successfully cleaned you can try again to open admin panel.