Restarting the Forms Routing Service resolved this for us. 

We became aware of the issue because we had one form that we noticed was not coming in.  When submitting a test via a link on our website (like the public would do) it appeared to be submitted as usual.  End event confirmation message etc.  However, no evidence of the form in "Forms" (nothing showed up in the Monitor tab).  

The first thing I did was to unpublish and republish the form.  I was then able to submit a test form successfully, that showed up in Forms (in the Monitor tab).

However, the forms that had been submitted as tests, were nowhere to be seen.  Found this posting and my coworker restarted the Forms Routing Service on the DMZ server (thanks John!).  Voila!  All the test forms (and some others that were queued up that we did not know were hanging) showed up. 

Turns out we had some maintenance over the weekend, that could have impacted our Forms processing.  The thought was possibly the DMZ server came back up before our Internal Forms server?  Passing that along in case it is useful.

The great news is that no forms were lost, they do go to the Routing Error holding place described above in John C's post.  They will continue on once the Routing Service is restarted.  At least they did for us.  :)

Thanks so much!  This post was a great help!

Christine