I voted to close Finding boundary co-ordinates from given set of point co-ordinates?
But now have been convinced otherwise. However I don't see a way to undo a vote to close.
Since July, 2013 it was implemented the functionality of retracting close votes. See more in the response from @animuson to:
More guidance is available within the Cast Close And Reopen Votes privilege:
- You can retract a close vote at any time if the question has not yet been closed by clicking on the 'close' link under the question. This will display the standard close dialog, with the submit button changed to a 'Retract Vote' button.
Original answer (which is obsolete):
There is no way to undo a vote-to-close. But close-votes age off after four days so, if they made a strong case to keep the question open, the post will automatically lose all of its close votes after a few days.
If the question was closed anyway, you can always vote to re-open it.