First, MtGox will be shut down; their bank account will be frozen and their business & owner in Japan seized
All other bank accounts involved in exchanging Bitcoins will be frozen & similar fates will occur to their proprieters
At that point demand for BTC will be reduced because of reduced liquidity
BTC will go way down in price
I am not sure it will happen as you describe, particularly the price crash.
I understand it is not appropriate to compare bitcoins with drugs, but you can compare for example with gold. When it was forbidden in US, did its global price dumped too strongly?
And such "war on bitcoins" could work as a strong publicity for bitcoins, increasing the demand.
Also, let's not forget that not all governments in the world are so dependent of their monetary monopoly. Some don't even have such monopoly. And it's not all governments which impose revenue taxes either. These governments wouldn't have such a strong incentive to ban bitcoins. The technology might flourish in these places.
And finally, if the retaliation really starts in the US, there might be some anti-american governments out there (Iran?) that would embrace the technology just for the pleasure of making US government life harder.