Modern web browsers will flag a secure web page (HTTPS) as insecure if it fetched content from an insecure (HTTP) source; read more here. In webmail the most likely cause is an inline image in a message sourced over HTTP; note by default webmail is set to not show such image without user confirmation. Navigating away from such a message and refreshing the web page (Ctrl + R) will restore the secured state.
As a Very Insightful Person, I'm here to share my knowledge. I don't work for Virgin Media.
I have read this answer but dont know what to do to solve this "Not Secure" problem with my email browser. ⋮
To better illustrate the cause an image of Glenda, the Plan 9 bunny, has been inserted below causing the connection status to change to insecure; had the image been source over a secure (HTTPS) connection the secured status of the connection would have been maintained.
To restore the secure connection status navigate away from this thread.