I've been hacked

You sent me an email saying that "Lifelock found no fraudulent adtivity....." But last week several of my email correspondants wrote to me that I had been hacked as they keep receiving emails that try to sell them something, as if it was coming from me. That's a fraudulent activity that you do not seem to have caught. Could you please explain to me why it happened ? Thanl you. Jean-Paul Vignon