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?
I've never heard anyone ever being refused a visa because another stamp in the same passport is unused. I can't prove a negative, of course, but if it were me, I wouldn't worry about it at all.
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.
Actually, I was be granted a Schengen visa and ended up not going for a personal reason. And the next time I asked for another visa I didn't get any question or rejection. Yeah don't worry about that at all.
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