1. Check that the DVD is clean and not scratched.
2. Check that the power LED on the front of the drive comes on when the DVD is inserted.
3. Check that the SATA and power cables are connected.
4. Check driver status in Windows Device manager.
5. All else fails, buy a new one, they're about $15 on newegg.
If these are those LG drives which I have had the uttermost displeasure of using then either buy a new drive or then go to the LG website, and get the latest firmware for that drive.
LG drives had this problem like 2 to 3 years back and they are even recognized as DVD RAM Drives. Double check with LG it'll help solve the issue.