Help Mac Air will not turn on...

My mum got a mac air for herself at Christmas and I have converted her to the wonders of mac, she's in love...

However she rang me today with this issue and not having an air myself and being miles away I don't really know how to help (yes I have told her to ring the helpline) but can anyone here help me help my distraught mother?

Here's the problem...

She was using the laptop this morning and it worked fine.
She closed it to put it into hibernation and then went a couple of hours later and it wont wake up.

I of course told her to plug it in, which she has done and she's now charged it until the green light on the charger is on. However it still refuses to turn on. She has held the on button for some time, she has pressed the space bar etc...

Does anyone know CPR for a mac book air?

Thanks

Suz