SQL query to update a table if user is blank

[NMend]

Hi Experts,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
<o></o>
I am using ADO and DAO to retrieve and send data from excel to access.<o></o>
I want to write a query where I want to update a field(User) in access table, if a user name is not updated before else it should pop a msg that this is already being used pls choose another and move to next available.

Can anyone help me in writing above query.

Thanks in advance for help. NMend