That's a problem with the NAS device. While some third-party NAS manufacturers claim to support Time Machine, I think it's risky to rely on them. Using a NAS with TM as your only backup is out of the question.
Most likely, the network location has changed, or the account set up on the NAS isn't seen as being the same one.
Try re-selecing it via Time Machine Preferences. You may need to either use WDs default password, or change it before doing that.
Also, I'm not sure what you mean by "trying to do another WHOLE backup." If you do a backup to an existing set of backups, TM will do an "incremental" backup of changes. You can't force a full one (and there's no reason to), short of deleting the backups, or if you've replaced or erased and restored your Mac's internal HD.