you could try using a console cableTo the nas ethernet port using putty, perhaps connecting a monitor directly to the NAS and try finding some diagnostic tools.

Maybe ipconfig /flushdns ipconfig /release ipconfig /renew

Are your shared directories still available? Are any other services this device provides still available? Is it registering on your network, with the router? Cable ok? Power supply good?

If the device won't ping using it's direct address (i.e. 10.0.0.2 or whatever your network is) then there's something wrong unless you've set it to not reply to pings. You can try to power cycle it, but be aware that the device may not boot properly at this point.

If you do reboot it and it comes back online, consider this your grace period.

The other comments are good. However, If you provide the model/maker it would help some.