Posting in an ancient thread, I know, but doing so to add information for anybody interested in the future.
I just had this same issue happen to my Zen V and the reason Greenflame's fix didn't work is because the electric switch ITSELF on the circuitboard of the Zen V is most likely broken. I took mine apart last night to verify this as my volume controls would not "click" nor would the volume change either. it seems that the "spring" inside the "volume +" switch on the motherboard is broken so once you push in on it switch, it will not pop back out, leading the player to think that you are constantly holding the "volume +" button down, meaning all other buttons will do nothing when pressed as the player still thinks you are holding that volume control - also, this obviously means it is impossible to control the volume. The only fix I know would be to have the player professional repaired somehow, or to somehow get a new switch and solder it on yourself, but that would be a massively finicky job unless you had very strong soldering skills (and I have no idea where you would get such a switch from...)