The Artlantis forums where Barry pointed you are the best place, otherwise file a tech support question with Abvent there. Be sure to list your OS as the issues are totally different Windows vs Mac. I just looked, and do not see a post from you on this issue there.
If you're on a Mac, your media should be here:
/Users/Shared/Abvent/Artlantis/Media
Check to see if the content is there. If not, then that's the problem.
If you have the Users folder moved to another disk drive, or have changed the permissions in some way, then there could be an issue due to that as Artlantis looks in a fixed path for the media. This can be changed in the Artlantis 5 Preferences - see attached, similar for Mac or Windows.
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB