If you always want the query to have a zero instead of a
field feom a table, just use 0 instead of the field name.
If you only want to display a zero when the query returns a
Null for a field's value, then set the form's text box's
Format property to a custom format. Without knowing what
the field would normally have in it, I can only provide an
example:
#,##0.00;(#,##0.00);0.00;"0.00"
See Format property in Help for other options.