Are you sure the poll hasn't been closed by someone other than you? Or you made it to close at a specific date and you've forgotten.
I'm sure all is well! The problem is the setting.
But I don't know what's wrong...
UP:
No one will help me?
any one person on this forum can't help me?
the same problem here mybb 1.6.4 ....
so no ideas?
Verify if the "timeout" value is "0", if not then you have set a number of days to timeout the poll and this could be the reason why it is actually closed. You can see this, if you click the edit link for your poll in its thread as an administrator. At the same time verify if the poll closed checkbox is not ticked!