Sounds like a breach of contract. In determining a party's rights, responsibilities, and to a certain extent remedies, under a contract it often helps to see the contract.
As regards the technical/pragmatic side, a website generally has 3 "components" - the domain name, the server (hosting), and the data.
There is potentially possession (which can ve slightly abstract for intangibles), ownership (legal and beneficial), legal ownership and beneficial ownership.
I would suggest that without more meaningful information we would struggle to provide more meaningful advice.