You can check the provider using open online resources, for example, https://lookup.icann.org
1. Submit your domain in the search field and click Lookup
2. Find the name in "Registrar Information" section. This is the company where you bought your domain.
Now you can go to its website and locate a login/sign in page to get into the domain admin panel. If you don't remember your login, try to contact the company's support for help.