User Profiles…Disappearing?

On my corporate network we’ve been having a problem with User Profiles recently. All user profiles are stored on our server and they’re setup to be “roaming” so users can log on from any PC on the network. The problem thats happening now is, certainuser profiles are disappearing off the server, or parts of them are disappearing. In other cases, the profiles are there, but when the user logs on none of their preferences are activated and their applications don’t work. Any and all help would be greatly appreciated.