Strange Problem

hi all

i am using flash from last 2-3 yrs, currently i am using
flash 8 where i am facing a strange problem

i have imported some jpg's and video's in flash and used them
in flash by making graphics and movie clips and saved my files
perfectly, next day when i opened my file and made some changes and
publish it to create swf some of the jpg's and videos are missing
then i checked the fla file from there also they are missing in
library only a black box is their with the same name(as of jpg's
and video's ) and also i am not able to update those files(option
is not activated) and this is not all when i am importing again
those files they are imported and there is two files in library
with the same name(flash not asked to replace the old file) plz
tell me what to do.