I have not contacted the registrar yet because I want to know precisely how to proceed.
What is your question? Please expand.
Are you looking for legal advice? Then we’ll move this to IMHO.
If you’re look for precise, you would need to contact a lawyer.
I’m going to take a wild guess here and suggest you didn’t own the domain name to begin with - you only assumed you did.
Often, not so scrupulous web hosting companies will register your domain for you, but they’ll do it under their name, with them as the owner. I found this out the hard way. When registering a domain one must be certain your name is attached to it as the owner and not Joe Blow’s Hosting. If you don’t it could be an expensive mistake if it’s a company and you have printed material (bus cards etc.) and other things attached to the name.
Not sure what steps you can take if you’re sure you actually own the domain. I suppose there’s civil action. First step is to talk to the person, if you can find them. In my case the guy absconded with lots of domains and money and couldn’t be found. He may have been using a fake name all along. It was years ago and I’m over it now. It cost me nearly $1,000 but cost others substantially more.
Start by looking up domain info: https://who.is/
Good luck!
First, please describe what you mean when you say they sold* it. How did you discover this and what specific information do you have?
When you register a domain name, your registrar is only the retailer. The name is actually registered with the registry (e.g., VeriSign is the registry for .com). It used to be that it was under your name but now most registrars offer masking of the registrant* for privacy purposes, but I don’t know what that means legally as far as who is the actually registrant. As @Keith1 says your first step is to see how your names were registered. Whois will tell you what information the registry has. For example, if you go to a third-party hosting company to get a turnkey web site, they may register your domain in their own name. It would be unethical and probably illegal, though, for a registrar to take your money and yet register the name to themselves.
*I don’t like to say “owner,” “bought,” and “sold,” because you are registering the name for a fixed term of time, not buying it.
(I worked for Network Solutions for a few years when they were the only place you could register a domain name, then VeriSign after they bought them.)
FWIW, I noticed that OP’s avatar image seems to incorporate a domain name string of (presumably) a web URL, so perhaps registration of that DNS entry is the point of contention.