MacBook Air keeps Re-Starting After Spinning Time Out

HELP!!!
My wife's Dual Core Intel 1.60GHz MacBook Air started performing slower and slower. Before I had the time to "investigate", she re-started the machine, and to our horror, has been doing nothing but since. [24 hours ago].
I ran the Extended Hardware Test. Said all was fine.
Did "it" hear her talking about replacing her with an iPad, and decide to punisher her? Machine is used for her work. Do we re-install system software?
Can this be done w/o losing files?
HELP!!!

The first thing I'd do would be to run Disk Utility from either the Recovery partition or from your system disc/flash drive (it would depend on the vintage of MacBook Air and also which version of OS X it runs).