When an account is blocked, the owner can no longer access their account, use it to create projects, or post new comments. When they login, they see a page that explains why the account was blocked.
Some blocks can be appealed: those users will see a web form they can use to request to be unblocked. If the owner can show that they understand why their account was blocked, and promises to follow the Scratch Community Guidelines in the future, they will be unblocked.
Some blocks cannot be appealed: for instance, some blocks automatically expire after a set amount of time, and alternate accounts that are blocked for being used while a main account is blocked must appeal their block on their main account. Those users will not see a web form and should not reach out to the Scratch Team regarding their blocked accounts.