I'll assume you already tried powering down the DNS-323; made sure the HDDs were firmly seated; and checked all wired connections.
You can try resetting the DNS-323 to the factory default settings. This will erase all your user defined settings, but will not impact your HDD configuration or data. Using a paperclip, press and hold the reset button on the rear of the DNS-323 for more than 5 seconds.
If this approach fails and you need to retrieve your data, you can try either mounting the HDDs in a Linux PC, or alternatively there are two popular Windows applications available that DNS owners have successfully used to mount HDDs in Windows PCs.