If you designed your query with the qbe grid, go back to design view and change your query type to a select query - Then see if you get any records. Note that your form will need to be open to get the textbox value, or else you will be prompted for a parameter.
To be sure you are actually referencing a form, use the expression builder to select the textbox on the appropriate form.
Format wise, =[forms]![formname]![txtboxname] should be fine.