I cannot see any of my contacts in BusyContacts, my address book is suddenly empty
In BusyContacts, the left panel displays a set of Address books that you have subscribed to. The checkbox next to the address book allows you to temporarily show or hide all contacts associated with it. If suddenly all your contacts appear to have disappeared, ensure you haven't toggled the address book's display OFF.
In case you have hidden unsubscribed address books from view, you may wish to show these first (BusyContacts View Menu > Show Unsubscribed Address Books).
Smart Filters can be toggled on and off with a single click. Ensure you're not inside of a Smart Filter view where the search criteria does not match any of your contacts and so all you see is an empty list.
More information on how to use Smart Filters can be found here:
Contacts gone. All contacts gone. Contacts won't show. Contacts not showing. Can't find contact.