Why did my Media Randomly go offline?

Hi, i had a perfectly edited session on my timeline i worked on for hours, i didnt move a folder or rename anything, all i did was save a session , i came back to it a hour later and i seen certain files that had questions marks and when i went to my timeline it played certain clips  with a red box saying media offline? but when i tried to locate the files so premeire can reconnect them i get a import encounter a generic error? i thought maybe the files became corrupted , but im able to play them on my desktop?

what gives?