Go to store.steampowered.com/account/ and click on "Change my password"
Go to steamcommunity.com/dev/apikey and click on "Revoke My Steam Web API Key
Go to store.steampowered.com/twofactor/manage and click on "Deauthorize all other devices"
Watch this youtube video it explains how your account was hijacked very well.
Message one of our admins on Discord and they'll be happy to help you.