OK. I've had this problem twice now in the last month. My late 2008 15" Macbook Pro with Yosemite runs fine all day long. I shut it down (normal shutdown), put it in my computer bag and do some things on my way home from work that end up taking 2-3 hours. Computer (in bag) sits in the back seat of my truck in temps that are around 10-15 degrees F. I get home, bring the unit in, plug it in and attempt to turn on. Nothing. The first time it happened I researched online and found that the SMC can cause this problem. So I followed the procedure (I have a removable battery in this unit), reset the SMC, hooked it back up and Voila! Came back on just fine. Second time, not so lucky. Reset several times with no effect. Left it sit on my dining room table overnight. (I keep my house at about 64 degrees F so not warm, but not really cold either.) Tried again next morning. No joy. Reset SMC again, still nothing. Figured this was a good time to try the Genius Bar at the local Apple store and made an appointment for mid afternoon. Tech looked it over, attempted the SMC reset again. Still no go. Took it into the back to open it up and do a more thorough check. Came back about 20 minutes later saying the insides looked very clean, no obvious problems, nothing that should be keeping the unit from turning on. Then she said since this was an old laptop that they don't carry parts for it anymore and couldn't offer any help. Then she suggested that since it was almost 7 years old maybe I should consider buying a new one... Well, of course I've considered it, but my wallet keeps telling me NO in no uncertain terms. Besides, this one was working just fine when it was running. I said thanks and brought it home. Decided to try one more time so I put it in the spare bathroom, shut the bathroom door and cranked the heater in there up to about 75 degrees F. Left it for about 4 hours, did another SMC reset, and it turned right on. Has anyone else had a similar problem to this and do you have any suggestions of things to try to keep this from happening again (aside from the obvious - don't let the computer get cold?) Any thoughts would be greatly appreciated.