How widely was this out? Did you pay for ads, submit to lots of sites? If so, I'd honor it, yank the book from other retailers, and put it in Select. Like, now.
But if you were just doing it on your own site or to your own fans, head over to Smashwords, put it up there if you haven't already, then either make it free there or generate a coupon to make it free. Then link everyone to there or publicize the coupon.
You can also go free on Kobo instantly, as well as iTunes if you put it up direct (not through another site like Smash, Lulu or D2D) and already have the book up. Price changes go through fast.
Just not Amazon or BN. Those are not easy venues to be free.
Good luck.