Yosemite Sleep Wake Failure
I have had this happen a few times myself.  If I was to guess, at the time you closed you lid on your computer you had the power supply hookup and running. After the lid was closed you removed the power plug.  Later you open the lid, without the power plug and it was a blank screen.  If this is your case, you have two options, 


unplug first before closing the lid
plug into power before you open the unit later. 


I have found if I give my Mac a few seconds after closing the lid, before removing the power, it will not freeze later when I open the lid. 

Still not sure of the problem, just a work around that I found after a little time with this type of problem.