I'm granted a one-entry Schengen visa and now I can't go. Will having an unused visa in my travel passport have any consequences like future visas refusal? Do I need to do anything special?
Not using it should not make much difference either way but using it and respecting its conditions (i.e. not overstaying/immigrating illegally) should weight positively in any subsequent application and help getting a multiple-entry visa. So if getting a visa was difficult for you, it could be a lost opportunity to establish yourself as a reliable visitor.
Only if you were planning to get married in that country. Getting a visa for purpose of marriage is a serious offense. Even in America some states make it a penalty to request marriage license then visa for your partner then not finish the task. I got hauled up once because i forgot to tell them I'm not using your document. Happy married now with visa. LOL