What operating system?
What version of After Effects?
Is this the trial version?
When you say that you import the file and "try to preview the file", what do you mean? Are you previewing in the Footage panel? Did you create a new composition, create a new layer based on this footage item, and preview the composition? Are you previewing the layer in the Layer panel?
Have these files worked for you before with your current setup?
Yepp, what the man said... Also tell us, what hardware. Could be as trivial as hardware-accelerated decoding getting stuck, because your graphics card isn't up to snuff or wrongly configured...