Maybe it's a cleaning issue, maybe it's a faulty drive. Give it a good cleaning-a really good cleaning-and try again.
Once the drive is all sparkly clean, if it still doesn't work with DVDs, find out if it will accept and read CDs. Every CD/DVD drive uses separate lasers to read (and to write) the different media, so it's possible that one laser has failed while another has not. If the drive won't accept CDs, that says it's something about the overall drive, but if it works with CDs and not with DVDs, that says it's one laser that's failed. Either way, it's a faulty drive.