MtGox is not the only exchange, although it might seem that way sometimes. It would be like your bank is closed on Sunday, but there's still banks from the same branch open next door.
Because the blockchain is shared, the only thing that mtgox being down prevents you from doing is buying/selling with money stored on mtgox itself-- you can still move money from your personal wallet(s) (you're not keeping 100% of your BTC on mtgox, right?) to another exchange and sell it there.
Because the blockchain is shared, the only thing that mtgox being down prevents you from doing is buying/selling with money stored on mtgox itself-- you can still move money from your personal wallet(s) (you're not keeping 100% of your BTC on mtgox, right?) to another exchange and sell it there.