What should I do if I feel my answer is correct and I am the only who has answered but it is not accepted?
Can you link to the question? If it's a new user who may or may not have come back to the site, there may be nothing you can do. If it's a regular user you can leave a comment asking if your answer solved their problem.
I've had answers accepted over a year after I left them, so sometimes you just have to be patient.