What kinds of chmod commands did you try? Do these two make any difference?
Code:
chmod +x helloworld
chmod 750 helloworld
Is there any possibility that your chmod command and ls command are acting on different files? e.g.
Code:
root@kali/~>chmod +x helloworld.c
root@kali/~>ls -l helloworld
would cause the situation you have now as the file whose permissions you are changing is not the one you are checking the permissions of (not that I haven't wasted hours in the past with similar typos at all, compounded by the convenience of recalling previous commands rather than actually typing them).

Sorry it's not really a solution but might help you step back and have an aha moment.