There are two possible reasons for that.
1. Password on your account had been reset.
In case someone gains the access to your email account, he will be able to reset your HitBTC password and get the access to it as well. To maintain high security level on HitBTC, once you initiate the password recovery request and successfully confirm it, the withdrawals on your account will be kept on hold for the next 72 hours. You will see the proper notification in your Account section. In case you haven't asked for password recovery, please contact our support team as soon as possible.
2. We have noticed suspicious activity on your account.
The main priority of HitBTC is the safety of your funds. In case there are reasons to believe that someone could gain illegitimate access to your account, we are able to restrict the withdrawal option on it. In case you would ever stumble upon it, please reach out for our support team. They will be glad to provide you with the next course of action towards safety and security.