Why don't you have steam guard on?
If your account is stolen you'll need to contact steam support here
https://support.steampowered.com/newticket.phpProvide the following information:
Steam account name for hijacked account
The original contact e-mail for the account
Select “Account Questions” as the category and “Hijacked or Stolen Account” as the subcategory.
Proof of purchase for the account as outlined below:
If retail software is registered to the account, attach a digital photo or scan of the CD Key registered to the account. Write your Support Ticket Number on the quick reference card or CD Key sticker below the code in permanent ink.
If a game was purchased through Steam with a credit card, please provide the following:
The Card Type (Visa, MasterCard, Discover, American Express)
Full Name on the Card
The last four digits of the credit card number - Do not provide the full credit card number
If a game was purchased through Steam using a PayPal account, please provide:
The Billing Name on the PayPal Account
The full Billing Address on the PayPal Account
The PayPal PayerID
The Invoice ID and the Transaction ID
It'll take about 2-3 days generally.
