|Do you know what make the dvd unit is?|
Is it a dvdrom only - or a burner too?
How old is it?
Is this a problem only with cdr - or cdrw too; and with all makes?
Have you tried cleaning it - using a dvd/cd lens cleaning kit; essentially a cd/dvd with a few brushes et on it... They can be bought for a few ££/$$$ most places -and of course eBay etc...
I seem to recall reading "somewhere" that cd/dvd units have two laser diodes - at least...; possibly one of these has failed in your unit?
Equally the laser(s) may have become misaligned... and that's not a road down which I would seek to travel in terms of resetting it... Replacement cd/dvd units are usually quite cheap these days. And one can of course use an external unit too - via a usb port?
And some serious reading all about the various systems: