More info would be needed on the error you get at boot.Is is just kernel panic?..This also sometimes happens when you update the kernel and the driver you have is for the old kernel and thus will no longer work, so the system wont see the raid.
You can try booting into the old kernel if the issue was you did a kernel update.
If you didnt update the kernel and your getting kernel panics.(Maybe a drive failed or the FS is degraded)
Boot into a live CD mount and check the integrity of the file system.
You can also try moving the raid to another system, just make sure you put the drives in the same order,the same ports and correct dirver .