Why would RAID 1 (Mirror) Fail with 2 Healthy HDD’s?

I have Server 2008 Standard 64bit installed with Exchange 2007 and a few other things. I am running a Intel server with 2 Intel Zeons and 2 x 750GB SATA drives in RAID 1 using the onboard raid controller. The intel board is the S5000PSL. Why would the RAID configuration go into a normal SATA configuration with 2 healthy drives? I am trying to find out what the possible causes could be.
One reason could be a power failure.