Discs eject or show blank disc inserted after Yosemite upgrade?

I am NOT an IT guru, so plain English answers if possible please...

I upgraded my mid 2010 iMac 21.5 to OS X Yosemite and now it won't read discs. It accepts them into the drive, whirrs up and down and then ejects cd's and dvd's, and shows 'blank disc inserted' with photo discs...

I've read so many other posts that are far to 'techie' for me to grasp, and I'm ready to sledge hammer the machine...

Any help? Anyone? Pleeeease!