The spell was broken. Remember what Grams told Bonnie after opening the tomb? She said that the tomb cannot be closed back, And that's why all the tomb vampires came out, I hope you remember Anna, her mom - pearl and all who were part of tomb vampires.
But soon, there was a spell which cast that if a vampire will go inside the tomb will not come back. [I dont remember who casted it but maybe it was Bonnie] and thats how Katherine was stuck inside the tomb (in season 2).. Remember how Elijah made the spell to lift up because Stefan was stuck there with Katherine? Then he compelled Katherine to stay in the tomb.
Funny fact is: Ktherine told Stefan NOT to kill Elijah because if he kills him, she'll be compelled to stay in the tomb forever. Turns out when Stefan temporarily killed Elijah, Katherine's compulsion was broken and she was free. She fooled him out.
hehehe that aweosme little bitch :P I love her more than Elena!