RAID 5 should still be accessible from your OS because is only has 1 of the 3 drives on degraded. But is weird that the OS is not booting up. I will recommend to check the boot options to ensure that you have the correct boot HD. Also what is the model of your server or motherboard, this will help me to provide you more information.
We need more information, motherboard model, operating system, are you able to access the operating system? Can you load the Intel® Rapid Storage Technology and check the status of the RAID?
If you are able to access Windows the first thing I recommend is to back all your information to avoi any lose of data.