Our paper was not published as part of the conference proceedings in IEEE Xplore, what should we do?
If you were reviewed and accepted, signed the appropriate forms, paid to attend the conference, and presented in an IEEE conference, then you should expect to be included in the proceedings. 

If you aren't, then most likely it is a result of a mistake, and can be corrected by contacting the conference publications chair.  The publications chair of the conference should have an IEEE publications contact, who has the right combination of authority and connections to help get it sorted out.  If they agree that you should be in the proceedings, then you can treat your paper as being published at the conference even if before the fix propagates to IEEE Xplore (which can sometimes take many months).  Importantly, this includes posting copies on your own site, where they can be easily accessed, per the standard IEEE copyright agreement.

On the other hand, if there is some reason that you actually did get dropped such that they are not willing to officially include your paper in the proceedings, then they are effectively relinquishing any claim on the paper and you may do with it as you wish, just as though it had been rejected in the first place.