If you have lost access to the account

Account Recovery
Make sure you are completely logged out of Steam before you start the recovery process

https://help.steampowered.com/en/wizard/HelpWithLoginInfo?issueid=406
https://support.steampowered.com/kb_article.php?ref=2347-qdfn-4366

When doing an account recovery, do not try to rush thru the process. Trying to rush thru normally results in information being missed and time wasted. Take your time and read everything.

Here is a guide that another Steam user was kind enough to make. It will help you navigate your way thru the recovery process if you are having any problems.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1126288560

Steam Account Recovery

It is possible to recover a Steam account. You don't need access to the email, phone or password currently tied to the account for this to work.

The recovery process allows you to contact Steam Support to provide proof of ownership.

Follow this guide to recover your account:

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3456188107

Support articles about account recovery:

Recovering a Stolen or Hijacked Steam Account

Providing Proof of Ownership

Link from the guide to start the recovery process:

https://help.steampowered.com/en/wizard/HelpWithLoginInfo?accountsearch=1&issueid=408&reset=1

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