Missing your deadline with no explanation is the point at which you're allowed to stop being nice.
Word your emails/PMs/messages politely, but be firm. Let your commissioner know that you would like your costume or a refund, don't leave the loophole of "I just want information" or they may take the easy way out and give you more excuses instead of either a costume or your money. And go ahead and give them an honest review anyway - because it serves as a word of warning to others, as well as opens the door for anyone else having problems with the same commissioner to either help you come to some sort of resolution, or find the courage to leave their own poor reviews so that no one else goes to this commissioner.
Even if life has seriously intervened and something tragic has happened, and they wouldn't be able to finish your costume, a good commissioner will email or message all of their customers to inform them that something has happened in their life that means there will either be a very long delay or no costume at all. And then do their best to offer refunds to those who still don't have costumes. Seeing this happen time and time again, I need to offer a word of warning to would-be commissioners as well as clients: if you want to be taken seriously, you must be forthright and up-front with your customers. If you can't do it, fix the situation. If you just don't want to do it anymore, then don't take commissions.
Founder and Vice-President, Madison Area Costuming Society, a chapter of the International Costumer's Guild
Strange Land Costuming - www.strangelandcostumes.com