You have only ever used one Apple account?
Read this support article about what to do after [or before] you change your Apple ID email address or password --> What to do after you change your Apple ID or password - Apple Support
Refer to this document for ways to contact Apple --> Choose your country or region - Official Apple Support
Select your country (also look for "other" regions), then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service by telephone --> Contact Apple for support and service - Apple Support