Bereavement - transfer of account

When my Dad died in July, we transferred the account to my Mum’s name. She has a new account number but are using the same email address as the old account. We have tried setting up a new online account but it won’t let us as it says an account already exists, and then when we try to do a password reset on that email address, we are not receiving the reset email link (it’s not reaching junk or anything)

please can you help us sort this out?