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You can use the Help site to try to self recover your account or contact Steam Support if you are unable to.
If you are unable to complete the self recovery process, Steam Support will verify the account is yours and recover it for you.
When contacting Steam Support try to provide as much of the following as possible:
Open the Steam client and go to Settings (Preferences on a Mac):
Select "Change Password...": 
Enter your current password:
(Don't remember your password? You can use the Help site to reset it)
An email from Steam Support will be sent to the email address associated with your Steam account. If you don't receive an email, make sure to check your spam folder.
Enter the 5 character code from the email and enter a new password. NOTE: It must not be a previous password and must meet the password strength requirements.
When you've completed changing your password, you'll receive confirmation by email. 