Accidental Pressing of CTRL+6 toggles hiding / unhiding of all objects which includes charts also. Pressing CTRL+6 will unhide all charts (objects) again.
Sometimes, Charts (objects) can be hidden through Selection Pane (Home tab>Find and Select>Selection Pane > This has Show All / Hide All buttons. In this case, you can show / hide appropriately.
Please note that both above are mutually exclusive. Hence, if use CTRL+6 to hide the objects, you can not use Show All of Selection Pane to show all objects and vice versa.