my singsang dvd player cant play dvds it only plays vcds any help

It may well have a failure.

DVD's and CD's use different laser beam widths/spacing and for this reason the DVD playback may have failed leaving CD functional.

Many times this starts with dual-layer discs no longer working and single layer still working fine. I would try a single layer disc for sure.

Some first generation bluray players actually had a full separate laser/pickup system(may still be this way, I have not opened a bluray player in a while) for dvd's so some dvd players may have done the same for cd playback, but I can not be sure on that one. Either way CD data is larger and more "easy to read" than DVD data.