How to fix the problem

HELLO
It is possible that the laser lens may need cleaning with a cotton bud and ispropyl alcohol (this cleans it properly). The lid of the machine will have to be removed to do this and I always maintain that if you don't know to do it get someone who can!
When you have done this try the machine again. If it works..great! If not then it's more than likely that the DVD part of the laser has failed. A laser unit in a DVD player has TWO lasers in one optical block (one for CD the other for DVD).
I wish you all the best with your problem and hope this has gone some way to helping.

There are 2 lasers, one for DVD and one for CDs. You can try cleaning the lens, with isopropyl alcohol and a cotton swab, VERY gently rubbing the lens. If this doesn't work then you likely will have an easier time buying a new unit. If the laser is not burnt out you can recalibrate it, but this requires a newer osciloscope and the ability to use it...

It's also possible you can find a new laser assembly for it, but it is usually cheaper to buy a whole new unit.