i would set it up something like this

public intCustomerID as integer

'Enter Event
intCustomerID = Me.txtCustomerID.Text

'Leave Event
If intCustomerID <> Me.txtCustomerID.Text Then
intCustomerID = Me.txtCustomerID.Text
'Call database and Load other Textboxes
End If

You can also check into the Validated and Validating Event. Then if
the customerID is no good return False so the user stays in the
txtCustomerID textbox.