WordPress display 500 Internal Server Error and you can’t access any pages including WordPress Admin Dashboard login page. Below is the detailed message you will see if you have an Internal Server Error.
Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, firstname.lastname@example.org and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
500 Internal Server Error show mostly caused by corrupted .htaccess file, file the .htaccess file and you should be able to access WordPress site again.
Step 1: Backup the current .htaccess file.
Step 2: Delete all the information in the .htaccess file and save it as a blank .htaccess file.
Step 3: WordPress site should be able to access it again.
Step 4: Log in to the WordPress Admin Dashboard to set the permalink structure again.
Step 5: Open the new working .htaccess file in an editor.
Step 6: Copy all the non-WordPress related .htaccecss information from old backed up .htaccess file eg. any search engine .htaccess information and IP blocks, etc.
Step 7: If you find that adding all the code from the previous .htaccess file is causing the same 500 Internal Server Error again then remove it and enter the information back to the working .htaccess file line by line to see what is causing the problem.