by DVD Player do you mean DVD Rom/R/RW drive ? or DVD playback software ?
If it's the drive I'd try pulling out the data cable (either IDE or SATA) and replugging it back in, could be that it's come loose. You could also try 'reseating' the power lead.
If it's the playback software I'd try uninstalling and reinstalling it. If that doesn't work try a free player like VLC or Media Player Classic.
If none of that works then you might want to think about formatting your HDD but seriously I think that's overkill for this problem. Course it's less expensive than buying a new DVD drive or new connection cables but it's a LOT more hassle.