Yes, you are right. Contacting customer service is not easy when you cannot log into the account. Consider using this form:
If it is not the right link, search for Upwork help center, search for
Reset Password and Security Question
and fill out the form on that page. It might help or at least someone will get back to you. if you remember the location and the approximate date then you might be able to get your account back.
I hope someone from the Upwork team will also contact you here.