500 errors occur due to causes internal servers, users usually less intervention.
1 A case is server error handling htaccess (eg file syntax is incorrect, or the server is not installed one module to handle some of its special directives: for example in htaccess RewriteRule which uses the server did not install such modRewrite ...).
2 This error usually occurs in one of the servers does not allow you to set permissions for the folders to CHMOD 777 mode, especially the admin or directory administrator (for the purpose of protecting parts of your back-end) . This error means that rarely happens on localhost, and when this fails you will not get into the admin page.
3 You are DoS attacks (one of the host server configuration when using the resources then this error)
Solution: go to File Mananger or use an FTP program on your host and CHMOD for folders and files, the folder is 755 files also should be 644.
If all else fails, try searching to see if any htaccess file, if so, if there are changes to tamthoi.htaccess not? If you change the name of all the possible causes 1