Hi there
First thing to do is to make sure you have a backup of your data. Remove the disk(s) with important data on and attach it to a SATA connector in a PC, or USB/SATA drive enclosure, or USB/SATA cable and in Windows install a Linux filesystem driver (http://www.fs-driver.org/) to mount your disks as read-only and access the files.
Is your NAS setup using RAID1 or do both drives contain different data?
Once your data is safely backed up, you could try resetting the NAS back to factory defaults and re-seating the drives after a power off.
Jamie