First off, the model you mention is the Photon III (3). The Photon II has one button and one switch (I believe). The Photon III has two buttons: one for momentary high mode, and the other for cycling through all the modes.

If one of the buttons does not work, the best thing to do would be to remove the battery cover and the battery. You will see the metal plate that says Photon 3 on it. Get a small screwdriver and lift that up. It will hinge away from the plastic housing along its attachment to the LED. Do not force it. You should be able to hinge the metal plate (and accompanying electronics) up enough to see the workings.

The buttons are simple metal spring-tab buttons that make contact to a solder spot on the IC board. Make sure the metal spring-tabs are not bent or otherwise screwed. Make sure there is no lint in there as well.

You can test whether the metal spring-tab buttons are making good contact by holding the battery to the metal plate and the side connector, then simply actuating the metal spring-tabs. They should work just like if you were pressing the plastic buttons.

If they do seem to work just fine, the IC board may have not been pressed tightly against the plastic housing. Reassemble and make sure the IC board/electronics and battery are inserted all the way against the plastic housing and that the battery covers fits tightly.

If the metal spring-tabs do not work reliably even when outside the housing, and they are not bent nor is there any lint stuck in there, try cleaning the metal with an eraser (carefully) or electronics contact cleaner spray to remove any oxidized metal that may be preventing electrical contact.

Other than those tips, just screw with it and see if you can get it to work. I've had a Photon 3 in my pocket for probably about 5 years now (still on the first generations LEDs, not the 2× brighter [Nichia CS] or even the new 2× brighter [Nichia DS] LEDs), and it has worked reliably for me.