Voting to reopen
You must have the "view close votes" privilege in order to vote to reopen your own question. Users with the "cast close and reopen votes" privilege can vote to reopen any closed question, not only their own. On established sites, these privileges are awarded at 250 and 3000 reputation points, respectively (beta sites have different thresholds).
From: "What limits are there on how I can vote? - Close/reopen votes":
"50 close/reopen votes per day per user on Stack Overflow, Mathematics, Server Fault, Super User and Ask Ubuntu, 12 close/reopen votes per day per user on Stack Apps, 24 close/reopen votes per day per user on all other sites (source)".
If you have the appropriate privilege, you will see a link that you can click in the post menu near the bottom of the question (on the same line as "Share⠀Edit⠀Follow⠀Flag"):
Clicking on this will ask you to confirm that you want to vote to reopen this question. If you cast the first reopen vote, your question will be added to the Reopen Votes review queue so that others can see it quickly and possibly cast more votes. After a certain number of reopen votes, the post will be reopened. The number of reopen votes required is the same as the number of close votes required to close on the site (five on most sites, three on some sites including Stack Overflow).
In some cases, a single vote is enough to reopen the question:
- A single vote from a moderator can reopen any question closed for any reason immediately.
- Questions closed as duplicates can be immediately reopened by a user with a gold tag badge for any of the question's tags.
- On Hardware Recommendations, a single vote from any user with the required reputation will reopen the question.
Comments to reopen
Leaving comments about voting to reopen does nothing in and of itself. You can try and address a specific user who may have left a comment as to why they voted to close and ask them to help cast reopen votes if you think they voted in error.
You can also leave a comment asking for other users to vote to reopen if you've edited your question to a better form.
Editing to reopen
If the comments are helpful as to why the question has been closed, you can edit the post so that it's a good, on-topic question, or to explain why it's not a duplicate. The close reason, while not always accurate, should point you in the right direction of what to fix.
Editing the body of a question after it gets closed may also add it to the Reopen Review queue, where people with the the ability to cast reopen votes will assess it. The question will only be added to the queue if you check the box to indicate that the question's original close reason has been addressed:
If the question is edited, reviewers will be shown a diff view of the edit by default. For this reason, you should make sure that any edits after closure should make clear why the question should be reopened. If you or someone else makes a minor edit and doesn't follow up with a more major edit, reviewers may only see the minor edit and thus disagree with reopening, which may force you to resort to the next option. (While edits where you don't check the box don't add the question to the queue on their own, these edits will be shown to reviewers if the question ends up in the queue otherwise, e.g. by someone voting to reopen.)
Requesting on meta to reopen
If you've tried the above options but it wasn't reopened, you can make a request on the per-site meta tagging it support reopen-request.
You should only do this if:
You tried the above three options and users still disagreed with reopening it, which will be indicated in the close notice and in the post history.
You can't exercise any of the above options (because the question is deleted or locked).
Clearly explain why the question should be reopened (why it's a good, on-topic question, or why it isn't a duplicate). This is also a good way to obtain further clarification about why your question was closed, beyond what's in the close notice.