The following are the few reasons which cause Internal Server Error and their possible solutions.
- The .htaccess File is Corrupt
Various things can cause your .htaccess file to become corrupt such as modifying it manually. In order to fix the corrupt .htaccess file, Take a backup of your existing .htaccess file and simply rename the file like .htaccessbak. If it fixed the problem then you can generate a new .htaccess file simply by changing permalinks in WordPress settings.
- A Plugin May be Causing Problem
To find out which plugins might be causing the issue, deactivate all of your plugins by changing the name of the plugins folder in your server as Pluginsbak and then reactivate them one by one by creating a new folder named Plugins and move the plugins folders from pluginsbak to Plugins One by One. Once you found a plugin which is causing a problem, you may try to uninstall and re-install the plugin, In some cases, you may need to completely remove the plugin and you may use any alternative plugin.
- Your Theme May be Causing Problem
If you see Internal server error immediately after activating a new theme. In that case, activate a different theme such as the WordPress default theme. You need to uninstall and re-install the theme, or simply find a different theme to use. You can also contact the developers of the theme and report the bugs in the theme.
- The wp-admin and wp-includes Folders Maybe corrupted
The wp-admin and wp-includes folders contain the files that make the WordPress Dashboard and other administrative functions run. In order to fix the problem, you may need to simply reinstall these folders. The reinstallation process is very simple.
- Backup your WordPress website in your local drive.
- Download WordPress from WordPress.org.
- Extract the "wp-admin" and "wp-includes" folders from downloaded WordPress Package.
- Upload the "wp-admin" and "wp-includes" folders and all of their subfolders via FTP to your web server and replace the existing wp-admin and wp-includes folder. This might fix the problem.