Hi and welcome to the WD community.
If one of the internal hard drives failed, you should be able to recover your files if you were using a RAID 1 configuration. You can try testing the internal hard drives one at a time by connecting them directly to a computer motherboard and running a DLG test. Be sure to connect the hard drives back to the same My Book slot. Check the links below for more information. It is not possible to use another My Book case to access your files
How to test a drive for problems using Data Lifeguard Diagnostics for Windows
Replacing a Drive (Page 19)