CNAME reuse issues

Hi,

Someone (not me) has used my subdomain to host their GitHub Pages. But I have the domain control with me and I am unable to host my GitHub Repo on it.

What should I do? I have also done:
CNAME subdomain.domain.tld myusername.github.io

But doesn’t seem to work. What should I do? Can someone please help?

Cheers,
Praveen