When using the Change username option the chosen username has to be a third party email account that you already have set up and have access to. Changing the username does not change the underlying email address.
To cancel the change sign back into My Virgin Media with the original username and password and follow the prompts.
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Assuming you are using VM web mail on a browser? Try logging out of the old account completely, then use an incognito window or a different browser and sign in to the new account. Does this help? If not, please get back to us, and describe in more detail, showing a screenshot if possible (but obscure any personal details, of course).