If you have a node label that gets deleted, for whatever reason, and if you had selected that node label when you ran another query your nodes may come up without a caption in the graph view. You'd still see the node labels in, for example, the table view but not the graph view. See below for an example:
On the far right hand of the screen, you'll see your possible captions. In this case, ID, born, and name. Clicking on any of those captions will make the labels visible on the graph as you can see in the screenshot below here:
If the above step doesn't work please open a support ticket.