Same as 2 posts down - it's in a CoDec that is not supported by AE or at the very least you have a problem with hardware accelerated decoding and playback on your system due to unsuitable or unsuitably configured graphics hardware. since you didn't provide any useful info, impossible to tell what is the cause at this point. Provide more info about the files and your system hardware and operating system.
As Mylenium says, we need to know a lot more if we're going to help you.
What version of After Effects? Have you installed the recent updates? What operating system? What is the codec used in the source footage? Has this ever worked before? What other software are you running? Do you have any third-party effects or codecs installed? Tell us about your computer hardware.