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Is it the internet connection of the Netgear router that has been lost?
Or is it the Nikon P600 camera's connection to the router that has been lost?
I am assuming the latter. The first thing to do in such a case, is to make sure that the Netgear router, by its indicator lights, not only has
1. an internet connection active
2. the WiFi facility enabled.
Then go through the standard steps to connect to the router with the P600.
If the connection fails three times over despite your entering all the parameters, then your router may have been reset, and its parameters for the WiFi connection, if so, are those of an Open connection with no password ***** - in which case, you can find out by entering no password (and also, the SSID that identifies the router will probably revert to the default value of 'netgear'.)