If you haven't already, plug the controller in using the USB cable.
If this doesn't work, press the reset button the back of the PS4 Controller (from memory I think it is on the right hand side), you will need a pin or something.
If this doesn't work, hold the Ps and share button until the light indicator on the back flashes quickly.
If none of these work, use a keyboard or borrow a ps4 controller, go to settings > devices > Bluetooth, then remove the dual shock controller. I usually restart my ps4 at this point however I don't think it is necessary, go back into the Bluetooth section, your playstation should now search for nearby Bluetooth devices, press the PS button on your controller.
I sometimes find you have to use a combination of the option and the Ps share button bit.