[TheUltraFish]

If you're not locked out of it, go in and immediately change your password as well as probably adding the additional authenticity check going into the future. Also check to see if they have bought anything, messed with your items, games, and friends list. If you have a credit/debit card attached to the account, I would check with your bank to see any odd purchases.

If you are locked out, ask friends to keep an eye on your account to make sure that the hacker isn't pretending to be you in games. The best you can do then is wait unless you can reset your password and wait for steam support.