I have provied them with all the documents they required, but they said my account is too high risk and they will not unblock it, I don't know what risk is it they are talking about or why after a year they have decided to lock my account without any prior notification.

Let this be a warning for anyone with their Localbitcoins account, they are changing policies I guess, this kind of thing happens all the time when businesses think they are big enough they start making new strict policies and become selective. They also forget those who were there for them when they were starting.