Basically, there is a book from my childhood that I vaguely remember, but do not know the title or author. So, I am going to put a list of details below. If you think you know the book, let me know in the comments.
- Set in modern day.
 - The last event in the book/series is a retelling of Ragnarok.
 - Main character is an original character.
 - Many side characters are taken from Norse mythology.
 - Loki is key, obviously, and there is a detailed description of his imprisonment.
 - It is probably from the children's fiction area (ages 8-12).
 - I owned the book about eight years ago, and I think it had been released fairly recently.
 - May also have been the last book in a series.
 - I think there is a plot point that it is all a video game, or virtual reality (this is uncertain).
 - An earlier book in the series took inspiration from the Greek Minotaur and labyrinth myth.
 
That's about all I remember. It isn't Percy Jackson, Ragnarok, Rune Marks, The Gospel of Loki or American Gods; before anyone suggests those.
Please if you have an inclining drop a comment below, it's really bugging me I can't remember the book.