I have a degraded RAID 5 but BIOS and Matrix Storage see all the drives up. Just under Volumes, RAID, Status is Degraded. And on BIOS bootup says Degraded. The RAID is still working when I access files. There isn't a rebuild/sync option either.
Each RAID system is different, but generally what you need to do is to identify which drive is dead and replace it with a new drive. At that point the RAID system should start to rebuild the set, or at least offer you the option to rebuild it.
Dealing with drive failures is something you really need to test with your RAID system BEFORE you put live data onto it. The last thing you want to do is to accidentally replace the wrong drive, since it could result in the loss of all your data.