It now only takes three votes to close or reopen a question. There is another question on the rationale and data behind changing from 5 to 3, so this is only an announcement.
If you see a question that should be closed or reopened, please vote accordingly. It matters more now than in the past! The button for this is close to the "Edit" button in the same panel. If you don't have the privilege to vote to close or reopen, you can flag the question and choose the close/reopen option, and then users with that privilege can see your suggestion in a review queue and act on it.
In the past most cases of closing and reopening have happened by a unilateral decision by a moderator. (Moderators can't vote like regular users on this matter. If they try, their vote is implemented immediately.) With this change in the system I would be happy to see regular users closing and reopening much more than in the past. Moderators will not stop entirely, but will hopefully take a smaller role in the future.
You are welcome to overturn any decisions made by moderators through this mechanism. That is both your right as a user and also serves as important feedback to us moderators. In the rare case that a question ends up being closed and reopened several times, moderators will have to step in and forcefully end the fight — but this is a bridge we shouldn't cross before we get there; I have seen this on other SE sites but never here.
If you have any questions or comments about these mechanisms (closing, reopening, flagging, moderator roles, and all that), feel free to discuss it in comments or answers below. Or ask a new meta question; no question is too small for that.