Most likely reason - you did not follow the step to "Finalize" the disk - check your DVD recorder's user manual to find the procedure for doing this.
Other possible reasons:
- the disk type you are using (for example, DVD+RW) may not be supported by your playback device. Check the specs of your playback device for the disk types it supports
- you have recorded an "HD Disc". These disks can only be viewed on an AVCHD compatible player (most Blu-ray players have this ability), and cannot be played back on standard DVD players
