A government ban is one of such scenarios. If the government makes the possession of Bitcoins a criminal offense, then very few people will be willing to take the risk.
I think that is not the thing that can bitcoin die because now there are some country which bitcoin is already ban but lots of people there are continuous collection bitcoin using vpn.
Even if country bans bitcoin people still can manage to earn them but what is the use of earning them if they cannot sell them or cash out them in their country as if its illegal then you will not find any trader who will involve in buying and selling of coins.
This is not a problem. You can always exchange your bitcoins online on the currency which is accepted in the banks in your country. For example dollar. A much bigger problem is that you have a few of them and you can't purchase with bitcoins.