What happened, before Aperture stopped importing movies? Did you install any other software lately, that might have changed video drivers or installed other video frameworks? Or did you run any helper applications to clean caches, etc?
The savest way to try to fix this would be to reinstall the system, but a quick and dirty way that helped in some cases is to restore a cache file, see
and also this thread: Re: Import video to Aperture library on Mountain Lion
I can not import a video anymore in Aperture 3.4.3.
You should provide more details on where the import is failing.
Are vidoes not showing up in the import window? Then make sure videos are not excluded from the import settings. See below.
Or, you can select the vidoes but for some reason they don't get into the library?
Or they are in the library but won't play?