I once got this problem that after updating WordPress (using IsleMag theme) accessing admin via wp-admin was suddenly not working anymore. Instead it gave Internal Server error 500.
Normally I always suspect Internal Server errors have to do with .htacess redirects not working properly. Renamed .htaccess. didn’t help. So it was time to troubleshoot.
Here is a list what to check in case of internal server error 500 in wp-admin:
1. Disable plugins. Internal server errors (error 500) are often caused by plugin or theme function conflicts. Since 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. Try switching to the Twenty Seventeen theme to rule-out a theme-specific issue (or another native WordPress theme). 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.
3. Rename .htaccess file. 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.
4. Check that WordPress file permissions are correctly set. All files need to be set 644 and folders need to be set 755. You can easily re-set them via FTP (i.e. with FileZilla) and selecting to apply permission first to only files and second to only folders.
See also related article: Solved: WordPress Admin Internal Server Error 500 after Update to 4.8
If this list does not help you solve the wp-admin “Internal Server Error 500” I would suggest you contact your hosting provider.