Find out who the registrant is
Look the domain up in the registry WHOIS: rotld.ro for .ro, domain.hu for .hu, denic.de for .de, nic.at for .at, and lookup.icann.org for .com and similar. Personal data is often hidden because of the GDPR, but you can usually see the registrar and expiry date. The registrar can confirm whether you are the registrant.
Collect your evidence
Gather paid invoices, the offer or contract, and any messages about the website and domain. Note the expiry date: if it is close, every week counts.
Ask calmly, in writing, for access and the transfer code
Send a short, polite e-mail asking for the logins or the authorisation code for the domain transfer (auth or EPP code), plus a copy of the website files. For .com and similar domains, ICANN rules require registrars to give the code to the registrant on request. Set a reasonable deadline.
Transfer to a registrar you choose
Open an account in your own name with a registrar, start a domain transfer and enter the code. Check first that no transfer lock is active. After the transfer, change every password and the contact e-mail straight away.
Back up the site and e-mail before moving
Make a copy of the website and the mailboxes. If you change the DNS servers without preparation, e-mail on the domain can stop for hours or even days.
If it can't be settled amicably
If the registrant refuses and you can prove you paid, talk to a lawyer. Some domain types also have dispute procedures. Everything here is general information, not legal advice.