I'd be inclined to offer the option of carrying the deposit over to a new date but if they wouldn't want to or are not interested in re-booking then I'd be insisting the booking fee is non-refundable as per T&C's. At the same time I would look to reword the T&C's for the future if they didn't already cover 'unprecedented circumstances' but I would have thought my terms from a few years ago would have already included 'circumstances beyond our control'. Either way I think I'd be covered if there were any challenges.