Just because indoor tables are not made to handle the outdoor elements does not mean there will not be accidents from time to time. More specifically, your table may suffer from water damage and do not worry as this is a common cause of damage for ping pong tables. The good news is you can repair the top but to do so, you must first allow the dampness to dry completely by either leaving it out in the sun (ensuring it is UV protected beforehand) or directing heat at it.
Either way, once it is fully dried you need to get your hands on a bonding agent such as PVA wood glue. But, before you apply the adhesive, peel away the surface that is damaged and clean away the now two exposed surfaces. Once that is done, go ahead and apply a small dose of the wood glue to each surface. To finish, apply lots of pressure to the two surfaces directly afterward.