I once got this problem that when updating my WordPress (it was using IsleMag theme) after the update accessing adming via wp-admin would not work anymore. Instead it gave error 500 – Internal Server Error.
Normally I suspect any internal server errors are caused by .htaccess. This time renaming the .htaccess didn’t help. So time to troubleshoot.
Here is a list of what to check in case of “internal server error” in wp-admin page:
1. Internal server errors (error 500) are often caused by plugin or theme function conflicts. Because you don’t have access to your admin panel, try manually resetting your plugins
If that resolves the issue, reactivate each one individually until you find the cause.
2. If that does not resolve the issue, try switching to the Twenty Seventeen (or some other native WordPress) theme to rule-out a theme-specific issue. Since you don’t have access to your Dashboard, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel (consult your hosting provider’s documentation for specifics on these), navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue.
If that does not resolve the issue, it’s possible that a .htaccess rule could be the source of the problem. To check for this, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, and rename the .htaccess file. If you can’t find a .htaccess file, make sure that you have set your SFTP or FTP client to view invisible files.