Unfortunately, there are too many components that can create this outcome on any dvd player to answer this question properly.
The two most likely culprits, however, would be the tracking mechanism or the laser and lense itself. As soon as you put in a dvd, the motor spins the dvd and the tracking mechanism moves the laser to the inside of the dvd. If either the track that the laser assembly is on, or the laser assembly itself is off center, you can get this error.
The laser assembly sits upon a set of 'rubber shocks' that can become worn with age causing the laser to point off a bit from center. If it is pointing off far enough (we're talking nanometers here), it can be pointing to far inward and not detecting any track to start from. Another part of the laser assembly is the optical pickup. If that is misaligned, the laser could be pointing correctly, but the reflected laser might not be hitting the optical pickup dead center. I would also point out that this lense, which detects changes in light, is the starting process of the rest of your player transforming the light into viewable data. If the lense doesn't see a reflection, the dvd player will probably figure that there isn't a disk.
Likewise, the tracking mechanism, if it is not working properly (such as not bringing the laser assembly to the inside of the disk), can also be the cause of your error. If the laser assembly doesn't work its way to center of the disk to find the starting tracks, it will not 'detect a disk.' As well, if it is pushing the assembly too far into the center of the disk, it could be causing the laser to focus on the blank center ring of plastic.
Sorry to say, you'll probably be taking the device in, or chucking it, depending upon the cost of the player. If it worked at one time, and not now, Even if it is a 'firm ware' problem, you can't update it without a disk.
If your player is VCD, SVCd pr jpeg cd compliant, see if you can play one. CDs have much wider 'tracks' compared to a dvd, if the '640' nanometer laser can read the wider cd tracks, then you very likely have a misaligned laser assembly.
Hope that helps.
Yangchu
April 2006