Do you have a backup of your data?
How were your drives setup, RAID 0, RAID 1, JBOD or standard disks. If they were RAID 0 or JBOD then with one bad disk you have lost your data. If it is RAID 1 then replacing the faulty disk and allowing the array to rebuild you should be able to continue using the unit.
Be aware that if you are using windows 10 it has some problems with its CIFS/SMB implementation which could be the reason you can't map the drive. Have you tried using 'Network' in windows explorer to find the NAS on the network and if so can you access the files there?
To check the integrity of your drives you need a USB/SATA caddy or adapter that has an external power supply and the disk manufacturers Disk Test Tools (usually a free download). Install the disk tools, attach the drive to test to a USB port and start the tests. This will show if the disk has a problem and, hopefully, what it is.