I've been there and even far worse than you. I used to be a big sports junkie and then I got osgood schlatter so my usual from dawn to dusk schedule of spending all my free time on sports had to be completely re-arranged and I didn't even have a PC back then.
What replaced sports in my life at that point was first and foremost books. Good old fashioned fantasy books. I was only a kid back then so I read a lot of short and nice books like Broken Sky, Harry Potter, Eragon and so forth. And then I moved into still short books but many and I started reading a series by an author called Margit Sandemo where I read 77 of the 82 books in the series (the last 5 were never translated from the other Nordic languages to neither Icelandic nor English so... never read them

) but perhaps at your age you'd like something more advlt like A Song of Ice and Fire (not that the children and teen books can't be nice when you're advlt too).
Then aside from books there are card games. My mom bought me my first pokémon cards the very day after I learned I had to quit sports which became a big hobby of mine, I then went into yu-gi-oh and currently I'm playing Magic: The Gathering which is a very fun card game if you like that and have some friends to play with. It's a rather expensive hobby though.
Then there is manga and anime. Some of the longer ones out there are the 3 biggest currently running ones. Naruto, Bleach and One Piece. They can all be enjoyed at any age but many would argue that One Piece is mostly for the younger audiences. You may also enjoy Azumanga Daioh if comedy and slice of life are for you and Berserk if you like more gritty stuff. The work done by Rumiko is also pretty famed two of the biggest works being Ranma 1/2 a slice of life comedy and InuYasha a adventure/comedy/slice of life/romance/omgwtfhowdoyoulablethis. I'd make a bigger list here but we got a pretty good anime thread that can inform you better. Manga is often better than anime though.
You could get engrossed in the MLP:FiM community

You could discover your latent modding powers and not play Skyrim, but make Skyrim!
Then there are always other games, on the PC or the console. If you got a PS2 then it's worth hunting for a game called Okami and another called Persona 3.