There are mainly two possibilities as to what is the problem:
1.) The drive is already quite old (bought my 708 back in 2004 IIRC) and is either dirty or damaged. You can try and clean the drive, there are numerous threads as how to clean a drive, just use search function. I myself have posted a small comment on how to clean just a few weeks ago.
2.) The discs are damaged. DVD media have the problem, that they are not made for eternity. A disc that was readable a yera ago or so may not be readable anymore now. It depends on media quality (Maxell is usually good) and burning quality (even a good media with bad burning can be unreadable as there was only a very tiny margin after the burning process that it could be read at all). 708 always had troble with different media types. I only used RICOHJPNR02. I don’t know what type your discs are, but if you havent checked burning quality after the bruning process it may be that the content is lost.
Either way just put the discs in an other drive and see if they can be read. Try to do a quality scan and if it turns out good your drive is faulty, if the other drives also have troble reading or the scans look bad your discs are gone.