The DVD laser lens/es are not reading DVDs.
FIrst thing to try is shutdown your iMac.
Use a credit card covered with a microfiber cloth insert it the long way into the drive slot and push it in and out a few times to remove any dust or lint in the drive.
Then start your Mac and try a DVD, again.
If still no joy, purchase and try a commercial "dry brush" CD/DVD drive cleaning disc and insert this into the drive and follow its instructions.
If still no joy, you'll need to purchase an external optical drive such as these.
Not all optical drives are Mac compatible.
Most Mac compatible drives you will only find to be purchased online.
Here are two on Amazon.com that are cheap and Mac compatible
http://www.amazon.com/LG-GP10NB20-Portable-External-Drive/dp/B004K2Z4MS/ref=sr_1 _3?ie=UTF8&qid=1398883289&sr=8-3&keywords=Mac+compatible+external+LG+optical+dr i ve
http://www.amazon.com/LG-Electronics-Portable-External-GP40NB40/dp/B009DS6OFU/re f=sr_1_7?ie=UTF8&qid=1398883289&sr=8-7&keywords=Mac+compatible+external+LG+opti c al+drive
Here's another source for Mac compatible optical drives. These are better quality optical drives.
These are also tray loading drives.
Tray loading optical drives are more reliable, plus they are capable of playing those smaller mini sized discs.
http://eshop.macsales.com/item/OWC/VLU2SD24XS/
http://eshop.macsales.com/item/OWC/MR3F82SD24S/
Good Luck!