I guess one could assume some personal problems causing an interruption in your transaction, or a complete end to it.
If you're in a hurry to get the artwork, go elsewhere and then, if the artist does come back you can request a refund, a different commission, or a reduced rate seeing that now you'll have two pieces of art when you only wanted one. If you're not getting any response, that sounds bad so I'd suggest you move on and hope s/he gets get back to you sometime but don't spoil your own project by waiting for something that may never happen. Expect the best, plan for the worst.
Heck, if it's not too difficult a project I'll have a go at the picture for you myself, gratis. I've a couple of daubs posted hereabouts - so that's how good/bad it'll be and I'd enjoy the challenge. Buy me a pint some day if you like it.
Or be sly and do an art comp here, or somewhere, with your brief and a fifty quid prize. Artists love winning things, and they love fifty quid notes, so that might be mad enough an idea to actually work.