We don't know anything about your system, your version of AE or the kind of footage you are importing. There is a current bug with some Microsoft WMV files and CS6 and CC, but there are literally dozens of things that could be causing your problem. Please give some details.
As Rick says, we need to know a lot more if we're going to help you. Please provide answers to all of the questions listed here: "FAQ: What information should I provide when asking a question on this forum?"