If you're unable to access the email address attached to your Steam account, you will need to raise a ticket via Steam Support - Regaining access to your lost Steam Account and resetting your account credentials.
While waiting your support ticket, choose *STEAM* as the product and Account Questions and Steam Guard/Email Verification as the categories.
Steam are going to ask for confirmation of who you are (Steam call this "Proof of Purchase"), this can normally be provided in the following forms. Please note: do not reply to this answer with any of the following information, this needs to be sent to Steam Support only;
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
If any other payment method was used you'll need to explain this in the support ticket.