The impact on the economy would be very negative assuming that the attack was both widespread and successful. The reason for this is that people's confidence in the banking system (and in much of the rest of the economy) would be badly shaken. They would worry that anything that uses the internet would be vulnerable to such attacks. They might, for example, not want to deposit money in banks any longer. They might also worry about investing their money for fear that it would be lost in attacks or that the companies they invest in would be damaged by attacks. With this loss of confidence, overall economic activity would decline and the economy would be hurt.