This is a serious issue and you should definitely create that DL again and make sure things are hooked up properly. This DL is where all messages that can't be delivered for any reason go (i.e. due to a full mailbox or too many attempts to connect to a mailstore that's off line for an extended period etc...). These messages will be lost entirely now while the system is in this state. That's bad.
You can rebuild the default objects (including the public DLs) by running the FixDefaultObjects.sql script - it was created for situations just like this (or where folks accidentally deleted the say goodbye handler or the like). You can run it using ConfigMgr.exe found under \commserver\. Select the "run database configuration script" option and then use the browse button to select the "FixDefaultObjects.sql" script then hit "run".
After you finish running the script you'll need to force Unity to sync with the directory to make sure that DL is in the directory properly again. You can do this by running "Setup /sync" from the \commserver\ConfigurationSetup directory. Just hit next and you'll see the fab pencil writing in a book for a while as it works.
That should put things back in a right state.
