This reminds me a bunch of:
http://en.wikipedia.org/wiki/Sword_Art_Online
Not necessarily a bad thing, but it's worth knowing about other narratives like your own so you can avoid cliches.
Maybe to clarify, you could say "when thousands of real people are trapped inside the world's largest MMO", or else "When thousands of human souls..." I think we all agree that you need some way to clarify it's not just their avatars that are stuck, or that they can't log off, it's actually their minds plugged into the machine. This inciting incident actually also takes care of explaining the stakes, because we get it from the outset, freeing up your word count for your character and action.
"game master" - just change to gamer. The fact that they're burnt out tells us that they have played a bunch. Also, you're wasting words explaining that she 'must learn' to play a game she evidently already knows how to play.
"hacker with a grudge" - vengeful hacker?
And maybe, once those elements are in place, just trim it back to suggest that the antagonists goal is destruction of the server?
"When thousands of real people are trapped inside the world's largest MMO, a jaded gamer must re-enter the digital realm to stop a vengeful hacker intent on destroying the entire server."