First, sit up and scream - that'll scare them! Actually, I'm not talking about when someone thinks you are dead in person. I am talking about when the powers that be report that you are dead, despite your very active pulse. In fact, being reported as dead is enough to get your blood pumping.
Here's a brief article about someone who had this problem - http://www.businessinsider.com/what-to-do-when-a-credit-bureau-believes-youre-dead-2012-4
The article gives some brief tips about what the alleged deceased should have done when she found out she was "dead". The problem with the article is that the steps given often do not fix the problem. So, if this should happen to you, take the article's advice and, if their advice does not work, hire an attorney experienced in Fair Credit Reporting Act cases to help you get the mess straightened out. You will likely even have a claim under the FCRA that could lead to you receiving compensation for the hassle the credit bureaus put you through. Feel free to contact me if this happens to you and I will either help you out or refer you to someone closer to you that can.