Internal server errors can be caused by a few different things. Please check the Error Logs in cPanel for specific information.
Here are the most common reasons for 500 errors:
(1) Bad permissions, Writable by group
Directories and folders should be 755. Executable scripts within the cgi-bin folder must be 755. Images, media, and text files like HTML should be 644.
(2) Bad .htaccess, Invalid command
In the (dot) htaccess file, you may have added lines that are either worded badly or conflicting. The best way to troubleshoot this is comment out the lines in the htaccess.
You can comment out a line in the .htaccess by adding # to the beginning. You are wise to save an original copy of any file before you make changes.
For example, if the .htaccess looks like
AddType application/x-httpd-php5 php
Then try something like this
#AddType application/x-httpd-php5 php
Broken lines and lines that start with php_flag are the most common mistakes. If you cannot determine which line is the problem, then comment out every line.