Part of my answer to What to do with questions whose OP hasn't visited the site for a long time? addresses the core question here.
###Is it answered in comments?
If there are comments that effectively answer the question, or that may answer the question and may be helpful to others, those comments should be (or should have been turned into) answers! In this situation it is appropriate for you to answer the question with information from the relevant comment(s). You can even quote them directly. Just make sure it's clear who said the valuable words (unless they've explicitly made clear they do not want you not to cite them, in which case, still make sure you're not passing off their words or solution as something that is solely your own work).
In this situation, it's common for short answers based on or quoting the comments of others to be posted as community wiki. I'm not aware of any consensus or policy requiring this, but it often makes sense for these to be CW answers because:
- CW status clarifies that the person who posted it is not necessarily its primary author.
- It's easier for users with lower reputation to edit CW posts, as only 100 reputation is required to edit without going through review. That's especially valuable here because often someone posts an answer in comments instead of as an answer when they feel it's not an adequate answer, or not detailed enough. Thus it's especially valuable for these answer to be easily expanded by anyone able and willing to do so.
- While you should not post any answer you think is meritless ("crap"), you might not want to associate yourself with this answer as strongly as usual. You might be unsure if it really works, for example. Posting it as CW sends less strong a message that it's your answer.
- You don't get unnecessary negative reputation for the answer. You've posted the answer as a service to the community, but it wasn't really your answer. Posted as CW, downvotes will have no effect on your rep.
- You don't get unnecessary positive reputation for the answer. Posting a valuable answer from comments is a useful service to the community, so it wouldn't be inherently bad for your reputation to increase as a consequence. However, you might feel you don't want to get reputation for it.
- Votes are entirely about the answer, and not at all about your contribution. This relates to the last two points. Since votes (up or down) have no effect on your reputation, people may be more willing to vote based solely on what they see as the value or usefulness of the answer.
- You can campaign for upvotes without violating etiquette. Upvotes on an answer helps remove a question from the "unanswered" list. (This is what people are usually talking about when they say "VTR" or "vote to resolve.")
- Of course, as in other situations, people may choose not to vote on any post; always remember that people's votes are their own.
- You still get recognition. Besides your name on the post (or in the edit history), and the community's knowledge that you helped clean up and resolve questions, remember the system awards badges for CW answers (same as regular answers).
- The revival badge is frequently awarded.
Sometimes it makes a good deal of sense for a post based on someone's comment not to be community wiki. If most of the content is your own, or you've extensively expanded, beautified, corrected, or customized the post. There's no clear line where it should or shouldn't be CW, and you're unlikely ever to be criticized for too many of your answers being community wiki. But you should feel free to post non-CW answers when you feel it's "your answer."