Probably because they went offline. So best answer is to see if there are any others who have same file & have available upload slots. Otherwise wait until they come back online. Put yourself in their scenario, are you willing to wait how many hours it may be needed for someone to upload a video file from you. Or will you log off just as per normal & leave it up to them to find you again another day. Personally I leave my computer on for 2-3 weeks at a time.