My private key (from myetherwallet.com) was stolen by hackers. Can I change it?
If you mean "change private key, keeping the same account address" - then NO. You should create new account and immedeately take an action to save your assets, controlled by old account (i.e. transfer money from old account to a new one).
If you mean "change stolen password, keeping the same account address" - then YES. You can recreate your keyfile with new password (programmatically). But it is not recommended, because hackers knowing the password, may be already know your private key as well.