Macbook Air (13" 2012) Fails to wake from sleep The Air doesn't wake up from sleep mode when the lid is closed. Have to do a hard restart by pressing the power button for 5 secs which isn't nice. Tried to select the startup disk from system preferences but it isn't helping. This behavior seems to have started after I started using the PGP disk encryption but I am not sure if anyone else has experienced the same problem.
Try resetting the Parameter Ram PRAM  Reset your computer’s PRAM .

And if that does not work try  Resetting the System Management Controller (SMC) 

If the problem still persists after trying the above contact Apple Support.