ACCOUNT HACKED

I have recently been getting notifications that I have exceeding my limit for sending emails, so my account has been blocked. As I only sent maybe a dozen a day was confused until I went into my "sent" box and found a whole load of junk/spam emails being sent from my account. Now it has been blocked and I cannot unblock it (I was getting a 'click here to unblock' link) but that isn't coming up any more. Please help as urgent now.