This may happen because there is a hiccup in Facebook. If this happens, please have patience because the statistics will be back soon :)
If it's just the Facebook insights, please also check the status of your Facebook channel. It could be that the channel is unauthorized and therefore, we're unable to collect any date from the channel. Only admins are able to check and change the status of a channel. To do so, they have to go to 'Manage settings', 'Channels', and find the channel you're looking for. If the channel is indeed unauthorized, the only thing you have to do is to re-add the channel. It may take a while to reload all the statistics after re-authorizing a channel.
If that's not the case, please send an email to firstname.lastname@example.org.