What happens to a spell if an oppenent runs out of mana while casting it?
What happens to a spell if an oppenent runs out of mana while casting it?


That's impossible. There is no chance to use spend mana between the point where you start casting the spell and the point where you pay for it.[1]


  but in response I played Twiddle and tapped one of his lands. 


"In response to" means "after it has been cast but before it resolves". When Ghor Clan Rampager is cast, the lands have already been tapped and the mana has already been produced.[1]

(Or did you mean in response to activating the land's ability? One cannot do anything in response to activating a mana source as it does not use the stack.)

You would have to cast Twiddle before he casts Ghor Clan Rampager (which means before he even mentions he's going to cast it), but then he could just tap the land for mana in response to you casting Twiddle, so it would have no effect.



Notes:


The steps to casting a spell or activating an ability are:

601.2a) Place card on stack.
601.2b-d) Make choices, including targets.
601.2e) Determine total cost.
601.2f) Activate mana abilities.
601.2g) Pay the previously determined cost.