***Note: I wrote this in Aug 13, 2016. It solves most of the issues related to the Nikon Coolpix P600. Take note you need to find the "replacing the camera mount" solution to do the repair for the Nikon Coolpix P600.
J Hess 04/2022
Most Helpful Answer
Jerry Hess @mrjlh
Posted: Aug 13, 2016
It is possible it is a small flex circuit that connects the power button module to the main circuit board is not connected correctly internally. If that is the case, it will need to be opened and the small flex circuit cable reset into its connector. If your camera is under warranty have it repaired through your dealer.
If it is out of warranty like mine was I was able to fix it on my own. If you are mechanically inclined here's how I fixed it.
I followed the disassembly procedure in "replacing the camera mount". You only need to get as far as step 9. The yellow flex circuits are inserted into a very delicate surface mounted PCB connectors. These micro connectors can not be repaired if broken. The entire PCB has to be replace. So take care when lifting the plastic connector lock that holds the flex circuit in place. What I found on my camera the flex circuit which goes to the "power switch module" was not inserted correctly or completely into the connector. It had come loose. I simply re-inserted the flex circuit securely back into the micro connector. I carefully inspected all other connectors at the same time.
I then inserted the battery and checked to see if my problem was solved. All functions on the power module worked. I removed the battery and then in reverse order put the camera back together. Be careful not put to much stress on the two yellow flex circuits. the short one is almost too short and Nikon could have made it another half inch longer.
Take care you get all the 11 screw holes aligned correctly. The rubber USB cover fell out and I almost panicked until I realized you only need to snap it back into the case. The screw attached to it merely holds the rubber cover once the camera is reassembled. It does not attach to anything else.
I hope this solves the problem. If not the power module may be defective and needs replacing.