If your Steam account has been hijacked, your computer's security is likely compromised as well.
Please reply to this ticket after completing these steps to confirm that your computer is secured and free of malware:
1) Scan your computer for viruses, key loggers, spyware, and other malicious code with a virus scan utility that has the most recent virus definitions.
2) Change your SteamPowered.com Support Account password and the password for the e-mail account you are using to contact Support. If you no longer have access to the email that is associated with your account, please reply with your new email address.
Finally, please make sure you have provided adequate information and proof of ownership: