I know I shouldn't be doing this, but it seems like a lot of people are getting their accounts hacked lately.
For whom that think their account is compromised, call support right away at 1800-4My-Xbox. Also, file a complaint on the user or contact the XBLPET Team if you didn't file a complaint. The Hotline for support is FREE. No need to worry about paying money, hence it is a 24/7 free hotline.
The only way someone has "hacked" you online is if they phished you to do so, such as them thriving/tricking you into giving them personal information, such as your email, password Etc.
Be safe, hence there are a lot of reports of "hacks". So please, if anybody threatens to "hack" you, take it seriously. File a complaint on the user, avoid the player, and block communications.