Can''t play dvd-r on my computer (by 2 users)

Is it the DVD player/cd burner combo on your computer or simple CD player?
Or
Compatibility: Check if your DVD player on your computer is compatible of playing -r's CD or DVD?.
Check the make and model of your computer and go to the manufacturer's website and find out from there.
Or
You might have to talk to the manufacturer's support.
and if you are sure that the DVD player and its compatible of playing -r's DVDs then first of all check by playing any +r's DVD and see how they respond. If you are certain that it plays - and +rs etc then recheck your recording of DVD's.
Also try installing a good program software like Nero on the computer which will give you options of controlling, burning and playing DVD/CD on your computer.
Best of luck!