I was reviewing "close" queue and on this review accidentally hit "leave open" button whereas I was actually meaning to click "close" - as the question is pretty much duplicate of the one suggested. What should I do in this situation?
You can open the question directly and revise your vote by clicking the "close" link under the question's text (in a somewhat counter-intuitive Windows-start-menu fashion, that's also where you will find the option to retract a close vote). If you cancel a close vote however, that's definitive, you cannot change your mind a second time (and, in particular, you can't retract your vote just to close with a different reason).
To find the question, you can obviously check if it's the home page or search for it based on what you remember but you can also find it through your review history. If you go to the main review page, there is a link reading "recent reviews >>" under each type of review. If you click it, you end up on a stats page but if you check the top-right of this stats view, you will see tree tab-like links. If you now click on "history", you will now see a list of all your reviews (or, if you have enough reputation, a list of everybody's reviews).