... to save your game just outside, and then exit the game. You then use Explorer to open the "Minisaves" subdirectory, and delete all the minisaves starting "Frosgard". This forces a reset of Frosgard to the initial position at the start of the game, with the exception of any completed quests (such as the one to open the harbour). So if you were able to get into Frosgard at any point during the game, you ought to be able to do it again now.Of course, if you weren't ever able to get into Frosgard at all, this won't help, and I apologise for wasting your time! 
