If the user account has been deleted from the NetCommunity website, the user will not be able to log in.  To undelete the user:
1. Log into NetCommunity 
2. Click on Users and Security, and select Users
3. Enter in the user's last name, and check the "Deleted" checkbox
4. Click Filter
5. Click the pencil icon next to the correct user account to edit
6. Click on the Hyperlink at the top to Undelete the user.  The text will read:

This user has been deleted.

You can undelete this user. This will also restore the link to The Raiser's Edge.

7. Click Save

You should now be able to log in as that user.