Ok, with all the files being in the same directory as the main 'send_it.php' program file, which I am assuming all sits at the root directory "/" of your server and there is no 'send_it' directory, use a text editor to modify the 'send_it.php' file and change the 'include' statements to read as:
include('./config.inc.php')
include('./email.inc.php')
include('./send_email.inc.php')
include('./send_mysql.inc.php')
include('./thankyou.inc.php')
So to clarify, for the above to have some success of finding the 'include' files, a directory listing of your Server root "/" would be expected to list the files:-
send_it.php
config.inc.php
email.inc.php
send_email.inc.php
send_mysql.inc.php
thankyou.inc.php
Another point to consider is that if the 'include' files have other references embedded within them to file directory paths in order to perform its function correctly and those file paths are incorrect, it will fail for the same reason you are currently encountering.
As such, best to use a text editor and check each of those files to confirm correct file paths.