This account may have been accessed by someone else.
Steam Support suspects your account may have been accessed by someone else. We are temporarily restricting modifications to this account as a security precaution. Please contact Steam Support so we can assist you.
How to regain access to your account:
Contact Steam Support and a support agent will verify that you are the owner of the account and restore account access to you. You may be asked to provide proof of purchase for games on the account or proof of ownership of the payment method.
More information is available here: Reclaiming a Hijacked Steam Account FAQ, Account Security Recommendations
This account will have restricted functionality until the owner has regained access.