I have had a few restaurants stop accepting certificates as you experienced Craig.
In every case, I was actually informed by restaurants.com of the change... and they offerred refunds/credits.
I also believe the OP misread the conditions of the certificate.... there is no certificate that I have ever seen at restaurant.com that you could get a $75 certificate for a $35 purchase. Or else there was a typo in the online certificate.... and restaurant.com notified to fix.