Without any a priori information about the similarity of the responders and the non-responders, you should assume that the non-responders have colluded to hide the effect from you as much as possible. This is not a random sample anymore.
You can still get information out of your sample of that form, but its very bad, and you should only pay attention to it if you can't afford to get anything better.
You can still get information out of your sample of that form, but its very bad, and you should only pay attention to it if you can't afford to get anything better.