If your WordPress wp-admin dashboard is not working but the front-end of your website is working, it could be because your wp-admin folder got corrupted. Here is how to fix it.
1. Find out which version of WordPress you are using.
Visit any page on the front-end of your website, right click and select ‘View Source’. Then search for the keyword ‘generator’ and you will find the wordpress version next to it.
<meta name=”generator” content=”WordPress 4.7.1″ />
2. Download the correct version of WordPress from WordPress Release Archive
Visit the WordPress Release Archive where you can download previous versions of WordPress and download the relevant version of WordPress (as determined in the above step).
3. Zip wp-admin folder and upload to your hosting file manager
From the downloaded WordPress file, copy and paste the wp-admin folder into another location. Then zip the wp-admin folder and upload the wp-admin folder to your file manager on your host.
4. Rename the existing wp-admin file to ‘old-wp-admin’
To ensure we can revert back to the previous wp-admin folder, rename the wp-admin folder to ‘old-wp-admin’.
5. Extract new wp-admin file
Extract the wp-admin zip file you uploaded. Once you extract the wp-admin file, you should have a new wp-admin folder with files in it.
If you are asked to define permission, choose 644 or 755.
And that should fix the problem.