WD NAS failed sto startup

You could try following the instructions here: https://support.wdc.com/knowledgebase/answer.aspx?ID=16778 

Please note that stage 3 of the instructions advises a low level format. Do not do this. Avoid doing anything that would write data to your RAID array as this could cause data loss.

In a situation where the data on the drive is critical and you have no backups, consider the value of the data and seek professional help if you cannot afford to lose it. If it were me, I would be taking each drive out of the unit and creating an image before doing anything. I would then attach the images to a virtual machine and attempt recovery of the images before doing anything with the NAS. Many NAS devices use Linux standard file systems so with a little research it should be possible to check the drives and hopefully reconstruct the array.

Dear Mr. Martyn,
many thanks for your replay. would you make me understand the steps of how to take each drive and make image of it, do you mean i take every HD from the enclosure of the NAS, attach it to any PC and take image of it?, also which software i should use for creating this image that allow me to attach it to virtual machine?
i really care about the data because it's very important
again, many thanks for your help

Firstly, if you really care about the data on the drives, it may well be better to hand it over to people who deal with it all the time. It may be costly, but you are giving your data the best chance. Please take my advice with caution and it is done at your own risk.

Firstly, you would be best to research the actual file system used by the NAS in question. As I said, many NAS devices use Linux based file systems and if this is the case, recovery may well be possible using Linux based tools. There are Windows based tools as well such as Active@ partition recovery that may be able to assist you, depending on the file system in use.

Essentially, to take an image of each of the drives, you need to remove it from the NAS and connect it to a different computer. Make sure that you maintain the correct drive orders so you know which bay each drive came from. Sometimes it does not matter, but at other times it does. Acquire an external drive bay and use a suitable drive imaging application to create an image of each disk. There are tools that can manage bad sectors if you find that the drives have errors. Note that errors could cause problems with rebuilding the array, but since you would be using disk images, your original drives remain untouched.

Do not under any circumstances initialise any of the disks. If the host OS taking the image does not recognise them, so be it. But, you are looking to do a block level copy of each drive. Ideally the imaging OS will recognise the partitions as RAID partitions, but it is possible that it won't. If at any stage you are unsure, stop and seek expert advice. I reiterate again, do nothing that will write to the disks.

Once you have an image of each drive, you should be able to mount the images as if they are "real" drives. The OS will not care that they are images. In an ideal world, the entire array will be recognised but you may need to scan the images for identifiable RAID array elements. Once you have the images and are able to mount them, feel free to post here again.

You don't need to use a virtual machine to mount the images, but it may make your life a little easier. Most modern OSes can mount images as if they were physical drives.

Dear Martyn,
Many thanks for your assistance, and as you advised me to be care about if the data is important and it really very important. :) and i'm working about searching for an vendor that can fix it without losing data. i was just need to know as an information for that it'll be possible to fix.
Tahnks

You're welcome. Best of luck with the data recovery.