No volume exists

EDIT: I now realize I may have misread your post, I thought you replaced the drives and was still seeing the same issue... But now I see that you're worried about having the same issue again. In this case two drives failed, that can happen with any device... The best thing to do is to have a proper backup plan, never store all your data on a single device. In addition to that, set up email alerts to notify you if drives are failing, this reduces the risk of having multiple disk failures as you can replace each disk as they start failing.

Original:

As per Hopchen's update the RAID is considered dead due to the two faulty drives, you say that you have now replaced the two drives.. Did you just take them out and put in new ones? Each drive would need to be replaced one by one and give the volume time to sync but as the volume is already considered dead this wouldn't work and you probably need to clone the drives (as Hopchen mentioned).. Did you clone both drives? 

Even after cloning the volume may be out of sync due to varying event counts between the drives in the array and would have to be forced back together... At this stage I think it's best to consider restoring from backup, or if you don't have it: Contact netgear support for data recovery options.