If you've already charged the camera and it's still not turning on, try removing the battery and cleaning the battery compartment. After this, leave the camera on the charger for a few more hours before attempting to turn it on again.
If after all this it's still not turning on, then the battery itself might be corrupt. You may have to purchase a new battery for the unit.
If you have any more problems, please refer to the [[Nikon Coolpix P90 Troubleshooting|troubleshooting page]].