The way you're talking I'd guess you actually mean you had your account stolen and not your MSN messenger program itself.
There is no such thing as a "hack or crack" for an identity.
A crack is a small, illegal program who's sole purpose is to trick another program (by editting, imputting a registration key, or some other method) into thinking it has been registered.
So unless you're trying to fool the MSN Messenger program into thinking it's registered you won't need a crack.
As for the hack, well there is no set term. Even if you're secret question was near impossible someone could still get into your account through your password. So you couldn't have had that hard a password to guess.
So my suggestion is just create a new account because that's all it is... an account.