Why would a Steam account be disabled?
Check the contact e-mail address for your Steam account to see if Steam Support has contacted you regarding the reason your account was disabled - accounts may be temporarily disabled pending verification of purchase information or account ownership.
Steam Support does not automatically contact the owners of disabled accounts in most circumstances.
There is a Zero-Tolerance policy for any violations of the Steam Subscriber Agreement and Online Code of Conduct. All accounts in a user's possession for any of the following activities will be disabled:
Piracy or Hacking
This includes using an unauthorized ("hacked") Steam client to access Steam, attempting to register fake CD Keys or attempting to register a CD Key which has been published on the internet.
Any fraudulent credit card use, credit card chargebacks, or Paypal chargebacks (regardless of when the transaction occurred).
Redeeming Fraudulent Gifts
Never accept a gift from an unknown user. Any accounts tied to a redeemed gift from a fraudulent source may be disabled.
Hijacking or Sharing Accounts
Do not use Steam accounts which you did not create.
Account "Phishing" and Deliberately Deceptive Activity
This includes contacting other users outside of Steam or over the Friends network under a misleading screen name or e-mail address (i.e. "Steam Admin", "VAC Administrator", "You Won Free Game Gift") and requesting password or account information.
Buying, Selling, or Trading Accounts
Accounts which have been bought, sold or traded will be disabled. This includes any other accounts in your possession at the time of the sale or trade, regardless of whether those accounts were also sold or not.
Violating Steam's Subscriber Agreement or Rules of Online Conduct
Please see the Steam Subscriber Agreement and Online Code of Conduct if you have any questions about the policies which govern Steam account use and account termination.