Cloud computing is ultimately the reliance on the internet for your computing needs; in a home consumer context that means storing you photos exclusively on a photo sharing service such as flickr and in a business context that means using crm services such as salesforce or maybe even hosting you applications in the cloud. The reasons for doing so are clear to understand, the home consumer can share photos with family and in some cases edit them using tools in these services (which would cost to buy for home uses). Businesses benefit by dispensing with server rooms and tin boxes and simply pay for what they use.
The risks of cloud are often discussed such as a a fire occurring at the data centre, a natural disaster occurring at the data centre, a terrorist attack on the data centre (physical or electronic) or even the financial collapse of the cloud provider.
Recent events in Egypt however have seen that a threat of the local government shutting down the internet during periods of social unrest is also likely. Even if this is unlikely to happen in Britain that is academic, as your cloud provider may be based in another country where these problems exist.
The result lost revenue for business (especially where internal processing systems have been farmed out to the cloud), inconvenience to home consumers and risk to cloud providers as their customer sees these risks materialise and take their applications back in-house.
Businesses for this and many other reasons need to have an exit strategy and or contingency for cloud failures in much the same way they would put in place disaster recovery (DR) plans for a traditional infrastructure.