What version of After Effects? I.e., is this the trial version? Have you installed the recent updates? What operating system? What kind(s) of source footage---sepcifically what file format and what codec?
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.
You need to provide much more info. What kind of video? What comp size? What computer hardware? Do you use OpenGL? Could be anything from an unsupported CoDec to graphics hardware acceleration problems, but on such sparse info nobody can tell you anything meaningful.
Afters Effects CS4, Yes I Think, I Haven't Updated Anything, Using Vista 32-Bit, MP4 Video.
Please answer all of the questions that I asked. I don't know which question your "Yes I think" answer is for.
If you're using the trial version, then note that the trial version doesn't support many formats. Specifically excluded are the MPEG formats. That is likely the problem that you're encountering.
Should I Change The Video Format To WMV/AVI Then Because I'm Using A Trial Version. Where Can You Buy CS4 Full Version?
When you go to the doctor, do you answer the doctor's questions to help him diagnose your illness?
So why don't you fully answer Todd's questions to help us diagnose your AE problem?
Until you do, here's what I think is wrong: After Effects doesn't like you any more. You did something to make it angry, so now you need to apologize and give it some flowers and a nice box of chocolates.