It sounds like one of your harddisks has crashed. I assume you have two harddisk, and that they are in a RAID failover setup (Raid 1 or Synology Hybrid Raid). If you don't have a failover RAID setup (RAID 0), you should copy all of your data to a computer as soon as possible!
Assuming you have a RAID failover setup on your NAS:
Login -> Program -> Storage Manager -> HDD Manager.
Does it say something like "Disk 1 has crashed"?
You could try to repair it, it should be a button on the called "Repair" in the Storage Manager.
This could take some time, but if it works your NAS should work normally again.
If this does not work, you must by a new harddisk (of the same size or bigger than the disk that crashed), and then replace the disk. You can then select Repair (from the Storage manager), and the NAS will rebuild itself.
See here for information about downgrading your NAS:http://forum.synology.com/enu/viewtopic.php?f=189&t=47006
(It could be risky, so save your data on a computer / USB drive before trying).