Yes, you can be sued for fraud without "official documentation"--there is nothing in the law requiring certain paperwork to establish fraud. (Fraud can be purely oral, for example.) However, you may not be guilty of fraud: fraud is an intentional misrepresentation (a knowing lie) made *before* someone does business with you, to induce or cause them to do business with you. A technical or other performance issue, if it did not involve a lie or misrepresentation, is not fraud; it could be breach of contract (failing to do or provide what you were supposed to), and so you could be sued to recover provable damages or losses directly caused by the lack of service, but based on what you write, that should be the most liability you are potentially exposed to. For example: a return of the amount for paid the number of weeks the site was down.