It could be any number of things... what year car is this? Are you trying to jump from the posts or 2004-5 auxiliary post? When was the last time it was running? Have you checked all the relevant fuses? When you put the leads on the posts are you getting interior power?What else could be going on? As Ive always been able to jump start cars in the past? Im just baffled is there a fuse that may have blown?